Techniques for controlling effective refractive index of gratings

ABSTRACT

A surface-relief structure comprises a surface-relief grating including a first material characterized by a first refractive index, a first layer of a second material having a second refractive index conformally deposited on surfaces of the surface-relief grating, and a second layer of a third material having a third refractive index conformally deposited on the first layer. The effective refractive index of the combination of the first layer and the second layer is less than, equal to, or greater than the first refractive index, thereby increasing the duty cycle and/or modifying the overall refractive index of the surface-relief structure. The first layer and the second layer are deposited using, for example, atomic layer deposition techniques.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims benefit of and priority to U.S.Provisional Patent Application Ser. No. 62/867,071, filed Jun. 26, 2019,entitled “Techniques For Manufacturing Slanted Structures,” thedisclosure of which is hereby incorporated by reference in its entiretyfor all purposes. This patent application is related to and is filedconcurrently with U.S. patent application Ser. No. ______, filed ______,and entitled “Techniques For Manufacturing Slanted Structures” (AttorneyDocket No. 1150246 (P009135US01)), the entire disclosure of which ishereby incorporated by reference into this application for all purposes.

BACKGROUND

An artificial reality system, such as a head-mounted display (HMD) orheads-up display (HUD) system, generally includes a display configuredto present artificial images that depict objects in a virtualenvironment. The display may show virtual objects or combine images ofreal objects with virtual objects, as in virtual reality (VR), augmentedreality (AR), or mixed reality (MR) applications. For example, in an ARsystem, a user may view both images of virtual objects (e.g.,computer-generated images (CGIs)) and the surrounding environment by,for example, seeing through transparent display glasses or lenses (oftenreferred to as optical see-through) or viewing displayed images of thesurrounding environment captured by a camera (often referred to as videosee-through).

One example of an optical see-through AR system may use awaveguide-based optical display, where light of projected images may becoupled into a waveguide (e.g., a substrate), propagate within thewaveguide, and be coupled out of the waveguide at different locations.In some implementations, the light of the projected images may becoupled into or out of the waveguide using a diffractive opticalelement, such as a straight or slanted surface-relief grating. Toachieve desired performance, such as a wide field of view, wide opticalbandwidth, high efficiency, low artifact, and desired angularselectivity, deep surface-relief gratings with large slanted angles,high refractive index modulation, and large grating duty cycles may beused, where the duty cycle is the ratio between the width of a gratingridge and the grating period. However, fabricating such deepsurface-relief gratings at a high fabrication speed and high yieldremains a challenging task.

SUMMARY

This disclosure relates generally to techniques for fabricatingsurface-relief structures, such as straight or slanted surface-reliefgratings. More specifically, and without limitation, disclosed hereinare techniques for fabricating deep structures having a wide range ofduty cycles (in particular, large duty cycles) and/or desired refractiveindex modulation in various inorganic or organic materials (e.g., metalalloy, silicon dioxide, silicon nitride, titanium dioxide, alumina,polymer), such as a slanted surface-relief grating used in awaveguide-based near-eye display system or a master mold fornanoimprinting the slanted surface-relief grating. Various inventiveembodiments are described herein, including methods, systems, devices,and the like.

According to certain embodiments, a surface-relief grating may include aplurality of grating ridges including a first material, and a layer of asecond material conformally deposited on the surfaces of the pluralityof grating ridges. A first region of the surface-relief grating ischaracterized by a first grating depth and a first duty cycle greaterthan a first threshold value. A second region of the surface-reliefgrating is characterized by a second grating depth and a second dutycycle lower than a second threshold value that is lower than the firstthreshold value. A difference between the first grating depth and thesecond grating depth is less than 20% of the second grating depth.

In some embodiments of the surface-relief grating, the first thresholdvalue is greater than 0.7 or greater than 0.8. In some embodiments, thesecond threshold value is lower than 0.5 or lower than 0.4. In someembodiments, the first threshold value is greater than 0.8 and thesecond threshold value is lower than 0.4. In some embodiments, athickness of the layer of the second material is less than 20% of aperiod of the surface-relief grating. In some embodiments, the firstregion of the surface-relief grating is characterized by a slant anglegreater than 30°. In some embodiments, the second grating depth isgreater than 100 nm. In some embodiments, the first region of thesurface-relief grating is characterized by a grating period less than200 nm.

In some embodiments of the surface-relief grating, the first materialmay include at least one of metal alloy, silicon, amorphous silicon,SiO₂, Si₃N₄, titanium oxide, alumina, TaOx, HfOx, SiC, SiOxNy, spin-oncarbon (SOC), amorphous carbon, diamond-like carbon (DLC), or an organicmaterial. In some embodiments, the second material may include at leastone of SiO₂, Al₂O₃, TiO₂, HfO₂, ZrO, ZnO₂, Si₃N₄, or an organicmaterial. In some embodiments, the second material has a higherrefractive index than the first material. In some embodiments, the layerof the second material is characterized by a variation in thickness lessthan 10% of an average thickness of the layer of the second material. Insome embodiments, the layer of the second material is conformallydeposited on the surfaces of the plurality of grating ridges by atomiclayer deposition or plasma-enhanced chemical vapor deposition (PECVD).

According to certain embodiments, a method may include imprinting oretching, in a first material layer, a surface-relief structurecharacterized by a minimum duty cycle and a maximum duty cycle less thana first threshold value. A first region of the surface-relief structurehas the minimum duty cycle and a first depth. A second region of thesurface-relief structure has the maximum duty cycle and a second depth.A difference between the first depth and the second depth is less than20% of the first depth. The method may further include depositing,conformally on surfaces of the surface-relief structure, a layer of asecond material to form a surface-relief device.

In some embodiments, the first threshold value is lower than 0.7. Insome embodiments, a maximum duty cycle of the surface-relief device isgreater than 0.75. In some embodiments, the layer of the second materialis conformally deposited on the surfaces of the surface-relief structureby atomic layer deposition (ALD) or PECVD. In some embodiments, themaximum duty cycle of the surface-relief device is greater than 0.7, aslant angle of the second region of the surface-relief structure isgreater than 30°, and the first depth is greater than 100 nm.

According to certain embodiments, a surface-relief structure may includea surface-relief grating including a first material characterized by afirst refractive index, a first layer of a second material characterizedby a second refractive index and conformally deposited on surfaces ofthe surface-relief grating, and a second layer of a third materialconformally deposited on the first layer, the third materialcharacterized by a third refractive index. One of the second refractiveindex and the third refractive index is lower or greater than the firstrefractive index, and an effective refractive index of a combination ofthe first layer and the second layer is equal to the first refractiveindex. In some embodiments, the surface-relief grating may include agrating imprinted in an organic material.

In some embodiments, a thickness of the first layer and a thickness ofthe second layer may be selected based on the first refractive index,the second refractive index, and the third refractive index. In someembodiments, the surface-relief structure may also include a third layerof the second material conformally deposited on the second layer, and afourth layer of the third material conformally deposited on the thirdlayer. In some embodiments, the surface-relief structure may alsoinclude an overcoat layer on the second layer, the overcoat layerfilling gaps in the surface-relief grating and characterized by a fourthrefractive index different from the first refractive index.

According to certain embodiments, a surface-relief structure may includea surface-relief grating including a first material characterized by afirst refractive index, a first layer of a second material conformallydeposited on surfaces of the surface-relief grating and characterized bya second refractive index greater than the first refractive index, and asecond layer of a third material conformally deposited on the firstlayer and characterized by a third refractive index greater than thesecond refractive index. In some embodiments, the surface-relief gratingmay include a grating imprinted in an organic material. In someembodiments, the surface-relief structure may also include an overcoatlayer on the second layer, the overcoat layer filling gaps in thesurface-relief grating and characterized by a fourth refractive indexgreater than or equal to the third refractive index.

According to certain embodiments, a surface-relief structure may includea surface-relief grating including a first organic materialcharacterized by a first refractive index, a first layer of a secondmaterial conformally deposited on surfaces of the surface-relief gratingand characterized by a second refractive index lower than the firstrefractive index, and a second layer of a third material conformallydeposited on the first layer, the third material characterized by athird refractive index lower than the second refractive index. In someembodiments, the surface-relief structure may also include an overcoatlayer on the second layer, the overcoat layer filling gaps in thesurface-relief grating and characterized by a fourth refractive indexlower than or equal to the third refractive index.

This summary is neither intended to identify key or essential featuresof the claimed subject matter, nor is it intended to be used inisolation to determine the scope of the claimed subject matter. Thesubject matter should be understood by reference to appropriate portionsof the entire specification of this disclosure, any or all drawings, andeach claim. The foregoing, together with other features and examples,will be described in more detail below in the following specification,claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments are described in detail below with reference tothe following figures.

FIG. 1 is a simplified block diagram of an example of an artificialreality system environment including a near-eye display according tocertain embodiments.

FIG. 2 is a perspective view of an example of a near-eye display in theform of a head-mounted display device for implementing some of theexamples disclosed herein.

FIG. 3 is a perspective view of a simplified example of a near-eyedisplay in the form of a pair of glasses for implementing some of theexamples disclosed herein.

FIG. 4 illustrates an example of an optical see-through augmentedreality system using a waveguide display according to certainembodiments.

FIG. 5 illustrates propagations of display light and external light inan example waveguide display.

FIG. 6 illustrates an example of a slanted grating coupler in awaveguide display according to certain embodiments.

FIGS. 7A-7C illustrate an example of a process for fabricating a slantedsurface-relief structure by slanted etching according to certainembodiments.

FIGS. 8A and 8B illustrate an example process for fabricating a slantedsurface-relief grating by molding according to certain embodiments. FIG.8A shows a molding process. FIG. 8B shows a demolding process.

FIGS. 9A-9D illustrate an example process for fabricating a soft stampused to make a slanted surface-relief grating according to certainembodiments. FIG. 9A shows a master mold.

FIG. 9B illustrates the master mold coated with a soft stamp materiallayer. FIG. 9C illustrates a lamination process for laminating a softstamp foil onto the soft stamp material layer. FIG. 9D illustrates adelamination process, where the soft stamp including the soft stamp foiland the attached soft stamp material layer is detached from the mastermold.

FIGS. 10A-10D illustrate an example process for fabricating a slantedsurface-relief grating using a soft stamp according to certainembodiments. FIG. 10A shows a waveguide coated with an imprint resinlayer. FIG. 10B shows the lamination of the soft stamp onto the imprintresin layer. FIG. 10C shows the delamination of the soft stamp from theimprint resin layer. FIG. 10D shows an example of an imprinted slantedgrating formed on the waveguide.

FIG. 11 is a simplified flow chart illustrating an example method offabricating a slanted surface-relief grating using nanoimprintlithography according to certain embodiments.

FIG. 12 illustrates an example of an ion beam etching system for etchinga slanted surface-relief structure according to certain embodiments.

FIG. 13A illustrates an example of etching a slanted grating. FIG. 13Billustrates an example of a slanted grating fabricated using an etchingprocess.

FIG. 14 illustrates an example of a reactive ion etching lag curve thatrepresents the relationship between duty cycles and etch depths ofsurface-relief structures etched using ME.

FIG. 15 is a flow chart illustrating an example of a method forfabricating a surface-relief grating with large duty cycles according tocertain embodiments.

FIG. 16A illustrates an example of a slanted surface-relief gratingfabricated using reactive ion etching or nanoimprint lithographyaccording to certain embodiments. FIG. 16B illustrates a first region ofthe slanted surface-relief grating of FIG. 16A that has been coated witha material layer according to certain embodiments. FIG. 16C illustratesa second region of the slanted surface-relief grating of FIG. 16A thathas been coated with a material layer according to certain embodiments.

FIG. 17A illustrates an example of a slanted surface-relief structurefabricated using nanoimprint lithography and coated with a firstmaterial layer according to certain embodiments. FIG. 17B illustratesthe slanted surface-relief structure of FIG. 17A that has been coatedwith a second material layer according to certain embodiments. FIG. 7Cillustrates an example of a stack of coating layers matching therefractive index of an imprinted surface-relief grating according tocertain embodiments.

FIG. 18A illustrates an example of a slanted surface-relief structurefabricated using nanoimprint lithography and coated with one or morethin material layers according to certain embodiments. FIG. 18Billustrates the slanted surface-relief structure of FIG. 18A that hasbeen coated with an overcoat layer according to certain embodiments.

FIG. 19A illustrates an example of a slanted surface-relief structurefabricated using nanoimprint lithography and coated with one or morethin material layers according to certain embodiments. FIG. 19Billustrates the slanted surface-relief structure of FIG. 19A that hasbeen coated with an overcoat layer according to certain embodiments.

FIG. 20 is a simplified block diagram of an example of an electronicsystem of a near-eye display according to certain embodiments.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated may be employed without departing from theprinciples, or benefits touted, of this disclosure.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION

Techniques disclosed herein relate generally to surface-reliefstructures, such as straight or slanted surface-relief gratings. Morespecifically, and without limitation, this disclosure relates totechniques for manufacturing surface-relief structures, such as straightor slanted surface-relief gratings used in a waveguide-based near-eyedisplay system. Techniques disclosed herein can be used to fabricatestraight or slanted surface-relief gratings with wide ranges of gratingduty cycles (in particular, large duty cycles), large refractive indexmodulation, small periods, small critical dimensions, high depths,and/or large slanted angles at a high fabrication speed and accuracy.The surface-relief gratings can be used as, for example, waveguidecouplers in waveguide-based displays to improve the field of view,increase the brightness or contrast ratio of displayed images, increasepower efficiency, and reduce display artifacts (e.g., rainbow artifacts)of the waveguide-based displays.

Gratings may be used in a waveguide-based near-eye display system forcoupling light into or out of a waveguide or for eye tracking. In somewaveguide-based near-eye display systems, the grating coupler mayinclude a straight or slanted deep surface-relief grating. In order toimprove the optical performance of the waveguide-based near-eye displaysystem, the grating coupler may need to have different diffractioncharacteristics at different regions of the grating. Thus, the gratingperiod, the duty cycle, the grating depth, and/or the slant angle of thegrating may need to vary across the grating. For example, slantedsurface-relief gratings with wide ranges of duty cycles, such as fromabout 10% to about 90%, can be very useful for optimizing thediffraction efficiency and/or the angular and/or spectral response ofthe grating. In addition, in some applications, to selectively coupledisplay light and ambient light into and out of the waveguide and intouser's eyes, improve field of view, increase brightness and efficiency,reduce display artifacts (e.g., rainbow artifacts), and/or improve otherperformances of a waveguide-based near-eye display system, a slantedsurface-relief grating having a wide range of grating duty cycles (e.g.,from about 0.3 to about 0.9), large slant angles (e.g., greater than30°, 45°, 60°, or larger), small grating periods (e.g., less than a fewmicrons or less than a micron), high depths (e.g., greater than 100 nm),and a certain refractive index modulation (e.g., Δn) profile may bedesired.

However, it may be challenging to fabricate such a slantedsurface-relief grating with a wide range of duty cycles (in particular,large duty cycles) and desired depths at a high production speed with ahigh fabrication accuracy and yield using current manufacturingtechniques, such as nanoimprint techniques or etching techniques. Forexample, it may be difficult to fabricate a deep slanted structure withlarge duty cycles using imprint techniques without cracking or breakingat least some grating ridges of the mold, stamp, or the imprinted deepslanted structure. To etch a deep surface-relief structure having a widerange of duty cycles using, for example, reactive ion etching (RIE), atleast some areas of the surface-relief structure where the desired dutycycles are large may have low etch rates and thus may not have thedesired depths due to the different etch rates at regions with differentduty cycles and/or periods (or pitches).

According to certain embodiments, to fabricate a nanostructure with aduty cycle range that includes large duty cycles (e.g., about 0.5 toabout 0.9), an initial nanostructure (e.g., a master mold or a grating)with reduced duty cycles (e.g., about 0.3 to about 0.7) may be imprintedor etched first, where the mask for the etching and/or the stamp for thenanoimprint may be adjusted to have duty cycles lower than the desiredduty cycles of the nanostructure. One or more layers of materials maythen be conformally deposited on the surfaces of the initialnanostructure to increase the duty cycles of the nanostructure. Forexample, one or more uniform layers of oxide (e.g., SiO₂, Al₂O₃, TiO₂,HfO₂, ZrO₂, ZnO₂, Si₃N₄, etc.) may be conformally deposited on thesurfaces of the initial nanostructure using techniques such as atomiclayer deposition (ALD) to increase the duty cycles of the nanostructure.In some embodiments, the materials of the deposited layers may haverefractive indices close to or higher than the refractive index of theimprinted or etched initial nanostructure.

In many implementations, the refractive index of the deposited materialmay not match the refractive index of the imprinted nanostructurebecause it may be difficult to find a material that can be depositedusing ALD techniques and also has a refractive index matching therefractive index of the polymer material of the imprinted nanostructure.According to certain embodiments, two or more layers of differentmaterials may be deposited on the imprinted nanostructure to increasethe duty cycle of the nanostructure and also match the refractive index.For example, a first thin ALD layer may include a first material havinga refractive index lower than the refractive index of the imprintednanostructure, and a second thin ALD layer may include a second materialhaving a refractive index greater than the refractive index of theimprinted nanostructure. The thicknesses of the first thin ALD layer andthe second thin ALD layer may be selected such that the effective indexof a combination of the two thin ALD layers may more precisely match therefractive index of the material of the imprinted nanostructure (e.g.,polymers). In some embodiments, the two or more layers of differentmaterials may include two or more sets of layers, where each set oflayers may include two or more layers of different materials.

Additionally or alternatively, the surface-relief structure may beimprinted or etched in a resin layer or other organic layer that mayhave a relatively low refractive index (e.g., resin or polymer with highrefractive index nanoparticles, such as TiO₂, GaP, HfO₂, GaAs, etc.).However, in many cases, it may be desirable that the surface-reliefstructure has a high refractive index in order to achieve a higherrefractive index modulation and a desired performance. According tocertain embodiments, a nanostructure may be imprinted or etched in a lowrefractive index material layer (e.g., resin or polymer layer), and oneor more sub-wavelength layers of materials having a higher refractiveindex, such as SiO₂, Al₂O₃, TiO₂, HfO₂, ZrO₂, ZnO₂, Si₃N₄, and the like,may be conformally deposited on the surface of the nanostructure toincrease the effective refractive index of the nanostructure.

In some embodiments, the ALD layers deposited on the imprintednanostructure may have refractive indices lower than the refractiveindex of the imprinted nanostructure to reduce the effective refractiveindex of the nanostructure. As such, when an overcoat layer having ahigher refractive index is formed on the nanostructure, the refractiveindex modulation (e.g., Δn) may be increased. In some embodiments, theALD layers deposited on the imprinted nanostructure may have graduallydecreasing refractive indices such that the refractive index modulationmay be apodized to, for example, reduce the side lobes and otherartifacts in the diffracted light beam.

In the following description, for the purposes of explanation, specificdetails are set forth in order to provide a thorough understanding ofexamples of the disclosure. However, it will be apparent that variousexamples may be practiced without these specific details. For example,devices, systems, structures, assemblies, methods, and other componentsmay be shown as components in block diagram form in order not to obscurethe examples in unnecessary detail. In other instances, well-knowndevices, processes, systems, structures, and techniques may be shownwithout necessary detail in order to avoid obscuring the examples. Thefigures and description are not intended to be restrictive. The termsand expressions that have been employed in this disclosure are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding any equivalents ofthe features shown and described or portions thereof. The word “example”is used herein to mean “serving as an example, instance, orillustration.” Any embodiment or design described herein as “example” isnot necessarily to be construed as preferred or advantageous over otherembodiments or designs.

FIG. 1 is a simplified block diagram of an example of an artificialreality system environment 100 including a near-eye display 120 inaccordance with certain embodiments. Artificial reality systemenvironment 100 shown in FIG. 1 may include near-eye display 120, anoptional external imaging device 150, and an optional input/outputinterface 140 that may each be coupled to an optional console 110. WhileFIG. 1 shows example artificial reality system environment 100 includingone near-eye display 120, one external imaging device 150, and oneinput/output interface 140, any number of these components may beincluded in artificial reality system environment 100, or any of thecomponents may be omitted. For example, there may be multiple near-eyedisplays 120 monitored by one or more external imaging devices 150 incommunication with console 110. In some configurations, artificialreality system environment 100 may not include external imaging device150, optional input/output interface 140, and optional console 110. Inalternative configurations, different or additional components may beincluded in artificial reality system environment 100.

Near-eye display 120 may be a head-mounted display that presents contentto a user. Examples of content presented by near-eye display 120 includeone or more of images, videos, audios, or some combination thereof. Insome embodiments, audios may be presented via an external device (e.g.,speakers and/or headphones) that receives audio information fromnear-eye display 120, console 110, or both, and presents audio databased on the audio information. Near-eye display 120 may include one ormore rigid bodies, which may be rigidly or non-rigidly coupled to eachother. A rigid coupling between rigid bodies may cause the coupled rigidbodies to act as a single rigid entity. A non-rigid coupling betweenrigid bodies may allow the rigid bodies to move relative to each other.In various embodiments, near-eye display 120 may be implemented in anysuitable form factor, including a pair of glasses. Some embodiments ofnear-eye display 120 are further described below with respect to, forexample, FIGS. 2-4 and 18. Additionally, in various embodiments, thefunctionality described herein may be used in a headset that combinesimages of an environment external to near-eye display 120 and artificialreality content (e.g., computer-generated images). Therefore, near-eyedisplay 120 may augment images of a physical, real-world environmentexternal to near-eye display 120 with generated content (e.g., images,video, sound, etc.) to present an augmented reality to a user.

In various embodiments, near-eye display 120 may include one or more ofdisplay electronics 122, display optics 124, and an eye-tracking unit130. In some embodiments, near-eye display 120 may also include one ormore locators 126, one or more position sensors 128, and an inertialmeasurement unit (IMU) 132. Near-eye display 120 may omit any of theseelements or include additional elements in various embodiments.Additionally, in some embodiments, near-eye display 120 may includeelements combining the function of various elements described inconjunction with FIG. 1.

Display electronics 122 may display or facilitate the display of imagesto the user according to data received from, for example, console 110.In various embodiments, display electronics 122 may include one or moredisplay panels, such as a liquid crystal display (LCD), an organic lightemitting diode (OLED) display, an inorganic light emitting diode (ILED)display, a micro light emitting diode (μLED) display, an active-matrixOLED display (AMOLED), a transparent OLED display (TOLED), or some otherdisplay. For example, in one implementation of near-eye display 120,display electronics 122 may include a front TOLED panel, a rear displaypanel, and an optical component (e.g., an attenuator, polarizer, ordiffractive or spectral film) between the front and rear display panels.Display electronics 122 may include pixels to emit light of apredominant color such as red, green, blue, white, or yellow. In someimplementations, display electronics 122 may display a three-dimensional(3D) image through stereo effects produced by two-dimensional (2D)panels to create a subjective perception of image depth. For example,display electronics 122 may include a left display and a right displaypositioned in front of a user's left eye and right eye, respectively.The left and right displays may present copies of an image shiftedhorizontally relative to each other to create a stereoscopic effect(i.e., a perception of image depth by a user viewing the image).

In certain embodiments, display optics 124 may display image contentoptically (e.g., using optical waveguides and couplers) or magnify imagelight received from display electronics 122, correct optical errorsassociated with the image light, and present the corrected image lightto a user of near-eye display 120. In various embodiments, displayoptics 124 may include one or more optical elements, such as, forexample, a substrate, optical waveguides, an aperture, a Fresnel lens, aconvex lens, a concave lens, a filter, input/output couplers, or anyother suitable optical elements that may affect image light emitted fromdisplay electronics 122. Display optics 124 may include a combination ofdifferent optical elements as well as mechanical couplings to maintainrelative spacing and orientation of the optical elements in thecombination. One or more optical elements in display optics 124 may havean optical coating, such as an antireflective coating, a reflectivecoating, a filtering coating, or a combination of different opticalcoatings.

Magnification of the image light by display optics 124 may allow displayelectronics 122 to be physically smaller, weigh less, and consume lesspower than larger displays. Additionally, magnification may increase afield of view of the displayed content. The amount of magnification ofimage light by display optics 124 may be changed by adjusting, adding,or removing optical elements from display optics 124. In someembodiments, display optics 124 may project displayed images to one ormore image planes that may be further away from the user's eyes thannear-eye display 120.

Display optics 124 may also be designed to correct one or more types ofoptical errors, such as two-dimensional optical errors,three-dimensional optical errors, or a combination thereof.Two-dimensional errors may include optical aberrations that occur in twodimensions. Example types of two-dimensional errors may include barreldistortion, pincushion distortion, longitudinal chromatic aberration,and transverse chromatic aberration. Three-dimensional errors mayinclude optical errors that occur in three dimensions. Example types ofthree-dimensional errors may include spherical aberration, comaticaberration, field curvature, and astigmatism.

Locators 126 may be objects located in specific positions on near-eyedisplay 120 relative to one another and relative to a reference point onnear-eye display 120. In some implementations, console 110 may identifylocators 126 in images captured by external imaging device 150 todetermine the artificial reality headset's position, orientation, orboth. A locator 126 may be a light emitting diode (LED), a corner cubereflector, a reflective marker, a type of light source that contrastswith an environment in which near-eye display 120 operates, or somecombinations thereof. In embodiments where locators 126 are activecomponents (e.g., LEDs or other types of light emitting devices),locators 126 may emit light in the visible band (e.g., about 380 nm to750 nm), in the infrared (IR) band (e.g., about 750 nm to 1 mm), in theultraviolet band (e.g., about 10 nm to about 380 nm), in another portionof the electromagnetic spectrum, or in any combination of portions ofthe electromagnetic spectrum.

External imaging device 150 may generate slow calibration data based oncalibration parameters received from console 110. Slow calibration datamay include one or more images showing observed positions of locators126 that are detectable by external imaging device 150. External imagingdevice 150 may include one or more cameras, one or more video cameras,any other device capable of capturing images including one or more oflocators 126, or some combinations thereof. Additionally, externalimaging device 150 may include one or more filters (e.g., to increasesignal to noise ratio). External imaging device 150 may be configured todetect light emitted or reflected from locators 126 in a field of viewof external imaging device 150. In embodiments where locators 126include passive elements (e.g., retroreflectors), external imagingdevice 150 may include a light source that illuminates some or all oflocators 126, which may retro-reflect the light to the light source inexternal imaging device 150. Slow calibration data may be communicatedfrom external imaging device 150 to console 110, and external imagingdevice 150 may receive one or more calibration parameters from console110 to adjust one or more imaging parameters (e.g., focal length, focus,frame rate, sensor temperature, shutter speed, aperture, etc.).

Position sensors 128 may generate one or more measurement signals inresponse to motion of near-eye display 120. Examples of position sensors128 may include accelerometers, gyroscopes, magnetometers, othermotion-detecting or error-correcting sensors, or some combinationsthereof. For example, in some embodiments, position sensors 128 mayinclude multiple accelerometers to measure translational motion (e.g.,forward/back, up/down, or left/right) and multiple gyroscopes to measurerotational motion (e.g., pitch, yaw, or roll). In some embodiments,various position sensors may be oriented orthogonally to each other.

IMU 132 may be an electronic device that generates fast calibration databased on measurement signals received from one or more of positionsensors 128. Position sensors 128 may be located external to IMU 132,internal to IMU 132, or some combination thereof. Based on the one ormore measurement signals from one or more position sensors 128, IMU 132may generate fast calibration data indicating an estimated position ofnear-eye display 120 relative to an initial position of near-eye display120. For example, IMU 132 may integrate measurement signals receivedfrom accelerometers over time to estimate a velocity vector andintegrate the velocity vector over time to determine an estimatedposition of a reference point on near-eye display 120. Alternatively,IMU 132 may provide the sampled measurement signals to console 110,which may determine the fast calibration data. While the reference pointmay generally be defined as a point in space, in various embodiments,the reference point may also be defined as a point within near-eyedisplay 120 (e.g., a center of IMU 132).

Eye-tracking unit 130 may include one or more eye-tracking systems. Eyetracking may refer to determining an eye's position, includingorientation and location of the eye, relative to near-eye display 120.An eye-tracking system may include an imaging system to image one ormore eyes and may optionally include a light emitter, which may generatelight that is directed to an eye such that light reflected by the eyemay be captured by the imaging system. For example, eye-tracking unit130 may include a non-coherent or coherent light source (e.g., a laserdiode) emitting light in the visible spectrum or infrared spectrum, anda camera capturing the light reflected by the user's eye. As anotherexample, eye-tracking unit 130 may capture reflected radio waves emittedby a miniature radar unit. Eye-tracking unit 130 may use low-power lightemitters that emit light at frequencies and intensities that would notinjure the eye or cause physical discomfort. Eye-tracking unit 130 maybe arranged to increase contrast in images of an eye captured byeye-tracking unit 130 while reducing the overall power consumed byeye-tracking unit 130 (e.g., reducing power consumed by a light emitterand an imaging system included in eye-tracking unit 130). For example,in some implementations, eye-tracking unit 130 may consume less than 100milliwatts of power.

Near-eye display 120 may use the orientation of the eye to, for example,determine an inter-pupillary distance (IPD) of the user, determine gazedirection, introduce depth cues (e.g., blur image outside of the user'smain line of sight), collect heuristics on the user interaction in theVR media (e.g., time spent on any particular subject, object, or frameas a function of exposed stimuli), some other functions that are basedin part on the orientation of at least one of the user's eyes, or somecombination thereof. Because the orientation may be determined for botheyes of the user, eye-tracking unit 130 may be able to determine wherethe user is looking. For example, determining a direction of a user'sgaze may include determining a point of convergence based on thedetermined orientations of the user's left and right eyes. A point ofconvergence may be the point where the two foveal axes of the user'seyes intersect. The direction of the user's gaze may be the direction ofa line passing through the point of convergence and the mid-pointbetween the pupils of the user's eyes.

Input/output interface 140 may be a device that allows a user to sendaction requests to console 110. An action request may be a request toperform a particular action. For example, an action request may be tostart or to end an application or to perform a particular action withinthe application. Input/output interface 140 may include one or moreinput devices. Example input devices may include a keyboard, a mouse, agame controller, a glove, a button, a touch screen, or any othersuitable device for receiving action requests and communicating thereceived action requests to console 110. An action request received bythe input/output interface 140 may be communicated to console 110, whichmay perform an action corresponding to the requested action. In someembodiments, input/output interface 140 may provide haptic feedback tothe user in accordance with instructions received from console 110. Forexample, input/output interface 140 may provide haptic feedback when anaction request is received or when console 110 has performed a requestedaction and communicates instructions to input/output interface 140.

Console 110 may provide content to near-eye display 120 for presentationto the user in accordance with information received from one or more ofexternal imaging device 150, near-eye display 120, and input/outputinterface 140. In the example shown in FIG. 1, console 110 may includean application store 112, a headset tracking module 114, an artificialreality engine 116, and eye-tracking module 118. Some embodiments ofconsole 110 may include different or additional modules than thosedescribed in conjunction with FIG. 1. Functions further described belowmay be distributed among components of console 110 in a different mannerthan is described here.

In some embodiments, console 110 may include a processor and anon-transitory computer-readable storage medium storing instructionsexecutable by the processor. The processor may include multipleprocessing units executing instructions in parallel. Thecomputer-readable storage medium may be any memory, such as a hard diskdrive, a removable memory, or a solid-state drive (e.g., flash memory ordynamic random access memory (DRAM)). In various embodiments, themodules of console 110 described in conjunction with FIG. 1 may beencoded as instructions in the non-transitory computer-readable storagemedium that, when executed by the processor, cause the processor toperform the functions further described below.

Application store 112 may store one or more applications for executionby console 110. An application may include a group of instructions that,when executed by a processor, generates content for presentation to theuser. Content generated by an application may be in response to inputsreceived from the user via movement of the user's eyes or inputsreceived from the input/output interface 140. Examples of theapplications may include gaming applications, conferencing applications,video playback application, or other suitable applications.

Headset tracking module 114 may track movements of near-eye display 120using slow calibration information from external imaging device 150. Forexample, headset tracking module 114 may determine positions of areference point of near-eye display 120 using observed locators from theslow calibration information and a model of near-eye display 120.Headset tracking module 114 may also determine positions of a referencepoint of near-eye display 120 using position information from the fastcalibration information. Additionally, in some embodiments, headsettracking module 114 may use portions of the fast calibrationinformation, the slow calibration information, or some combinationthereof, to predict a future location of near-eye display 120. Headsettracking module 114 may provide the estimated or predicted futureposition of near-eye display 120 to artificial reality engine 116.

Headset tracking module 114 may calibrate the artificial reality systemenvironment 100 using one or more calibration parameters, and may adjustone or more calibration parameters to reduce errors in determining theposition of near-eye display 120. For example, headset tracking module114 may adjust the focus of external imaging device 150 to obtain a moreaccurate position for observed locators on near-eye display 120.Moreover, calibration performed by headset tracking module 114 may alsoaccount for information received from IMU 132. Additionally, if trackingof near-eye display 120 is lost (e.g., external imaging device 150 losesline of sight of at least a threshold number of locators 126), headsettracking module 114 may re-calibrate some or all of the calibrationparameters.

Artificial reality engine 116 may execute applications within artificialreality system environment 100 and receive position information ofnear-eye display 120, acceleration information of near-eye display 120,velocity information of near-eye display 120, predicted future positionsof near-eye display 120, or some combination thereof from headsettracking module 114. Artificial reality engine 116 may also receiveestimated eye position and orientation information from eye-trackingmodule 118. Based on the received information, artificial reality engine116 may determine content to provide to near-eye display 120 forpresentation to the user. For example, if the received informationindicates that the user has looked to the left, artificial realityengine 116 may generate content for near-eye display 120 that mirrorsthe user's eye movement in a virtual environment. Additionally,artificial reality engine 116 may perform an action within anapplication executing on console 110 in response to an action requestreceived from input/output interface 140, and provide feedback to theuser indicating that the action has been performed. The feedback may bevisual or audible feedback via near-eye display 120 or haptic feedbackvia input/output interface 140.

Eye-tracking module 118 may receive eye-tracking data from eye-trackingunit 130 and determine the position of the user's eye based on the eyetracking data. The position of the eye may include an eye's orientation,location, or both relative to near-eye display 120 or any elementthereof. Because the eye's axes of rotation change as a function of theeye's location in its socket, determining the eye's location in itssocket may allow eye-tracking module 118 to more accurately determinethe eye's orientation.

In some embodiments, eye-tracking module 118 may store a mapping betweenimages captured by eye-tracking unit 130 and eye positions to determinea reference eye position from an image captured by eye-tracking unit130. Alternatively or additionally, eye-tracking module 118 maydetermine an updated eye position relative to a reference eye positionby comparing an image from which the reference eye position isdetermined to an image from which the updated eye position is to bedetermined. Eye-tracking module 118 may determine eye position usingmeasurements from different imaging devices or other sensors. Forexample, eye-tracking module 118 may use measurements from a sloweye-tracking system to determine a reference eye position, and thendetermine updated positions relative to the reference eye position froma fast eye-tracking system until a next reference eye position isdetermined based on measurements from the slow eye-tracking system.

Eye-tracking module 118 may also determine eye calibration parameters toimprove precision and accuracy of eye tracking. Eye calibrationparameters may include parameters that may change whenever a user donsor adjusts near-eye display 120. Example eye calibration parameters mayinclude an estimated distance between a component of eye-tracking unit130 and one or more parts of the eye, such as the eye's center, pupil,cornea boundary, or a point on the surface of the eye. Other example eyecalibration parameters may be specific to a particular user and mayinclude an estimated average eye radius, an average corneal radius, anaverage sclera radius, a map of features on the eye surface, and anestimated eye surface contour. In embodiments where light from theoutside of near-eye display 120 may reach the eye (as in some augmentedreality applications), the calibration parameters may include correctionfactors for intensity and color balance due to variations in light fromthe outside of near-eye display 120. Eye-tracking module 118 may use eyecalibration parameters to determine whether the measurements captured byeye-tracking unit 130 would allow eye-tracking module 118 to determinean accurate eye position (also referred to herein as “validmeasurements”). Invalid measurements, from which eye-tracking module 118may not be able to determine an accurate eye position, may be caused bythe user blinking, adjusting the headset, or removing the headset,and/or may be caused by near-eye display 120 experiencing greater than athreshold change in illumination due to external light. In someembodiments, at least some of the functions of eye-tracking module 118may be performed by eye-tracking unit 130.

FIG. 2 is a perspective view of an example of a near-eye display in theform of a head-mounted display (HMD) device 200 for implementing some ofthe examples disclosed herein. HMD device 200 may be a part of, e.g., avirtual reality (VR) system, an augmented reality (AR) system, a mixedreality (MR) system, or some combinations thereof. HMD device 200 mayinclude a body 220 and a head strap 230. FIG. 2 shows a top side 223, afront side 225, and a right side 227 of body 220 in the perspectiveview. Head strap 230 may have an adjustable or extendible length. Theremay be sufficient space between body 220 and head strap 230 of HMDdevice 200 for allowing a user to mount HMD device 200 onto the user'shead. In various embodiments, HMD device 200 may include additional,fewer, or different components. For example, in some embodiments, HMDdevice 200 may include eyeglass temples and temple tips as shown in, forexample, FIG. 2, rather than head strap 230.

HMD device 200 may present to a user media including virtual and/oraugmented views of a physical, real-world environment withcomputer-generated elements. Examples of the media presented by HMDdevice 200 may include images (e.g., 2D or 3D images), videos (e.g., 2Dor 3D videos), audios, or some combinations thereof. The images andvideos may be presented to each eye of the user by one or more displayassemblies (not shown in FIG. 2) enclosed in body 220 of HMD device 200.In various embodiments, the one or more display assemblies may include asingle electronic display panel or multiple electronic display panels(e.g., one display panel for each eye of the user). Examples of theelectronic display panel(s) may include, for example, a liquid crystaldisplay (LCD), an organic light emitting diode (OLED) display, aninorganic light emitting diode (ILED) display, a micro light emittingdiode (mLED) display, an active-matrix organic light emitting diode(AMOLED) display, a transparent organic light emitting diode (TOLED)display, some other display, or some combinations thereof. HMD device200 may include two eye box regions.

In some implementations, HMD device 200 may include various sensors (notshown), such as depth sensors, motion sensors, position sensors, and eyetracking sensors. Some of these sensors may use a structured lightpattern for sensing. In some implementations, HMD device 200 may includean input/output interface for communicating with a console. In someimplementations, HMD device 200 may include a virtual reality engine(not shown) that can execute applications within HMD device 200 andreceive depth information, position information, accelerationinformation, velocity information, predicted future positions, or somecombination thereof of HMD device 200 from the various sensors. In someimplementations, the information received by the virtual reality enginemay be used for producing a signal (e.g., display instructions) to theone or more display assemblies. In some implementations, HMD device 200may include locators (not shown, such as locators 126) located in fixedpositions on body 220 relative to one another and relative to areference point. Each of the locators may emit light that is detectableby an external imaging device.

FIG. 3 is a perspective view of an example of a near-eye display 300 inthe form of a pair of glasses for implementing some of the examplesdisclosed herein. Near-eye display 300 may be a specific implementationof near-eye display 120 of FIG. 1, and may be configured to operate as avirtual reality display, an augmented reality display, and/or a mixedreality display. Near-eye display 300 may include a frame 305 and adisplay 310. Display 310 may be configured to present content to a user.In some embodiments, display 310 may include display electronics and/ordisplay optics. For example, as described above with respect to near-eyedisplay 120 of FIG. 1, display 310 may include an LCD display panel, anLED display panel, or an optical display panel (e.g., a waveguidedisplay assembly).

Near-eye display 300 may further include various sensors 350 a, 350 b,350 c, 350 d, and 350 e on or within frame 305. In some embodiments,sensors 350 a-350 e may include one or more depth sensors, motionsensors, position sensors, inertial sensors, or ambient light sensors.In some embodiments, sensors 350 a-350 e may include one or more imagesensors configured to generate image data representing different fieldsof views in different directions. In some embodiments, sensors 350 a-350e may be used as input devices to control or influence the displayedcontent of near-eye display 300, and/or to provide an interactiveVR/AR/MR experience to a user of near-eye display 300. In someembodiments, sensors 350 a-350 e may also be used for stereoscopicimaging.

In some embodiments, near-eye display 300 may further include one ormore illuminators 330 to project light into the physical environment.The projected light may be associated with different frequency bands(e.g., visible light, infra-red light, ultra-violet light, etc.), andmay serve various purposes. For example, illuminator(s) 330 may projectlight in a dark environment (or in an environment with low intensity ofinfra-red light, ultra-violet light, etc.) to assist sensors 350 a-350 ein capturing images of different objects within the dark environment. Insome embodiments, illuminator(s) 330 may be used to project certainlight pattern onto the objects within the environment. In someembodiments, illuminator(s) 330 may be used as locators, such aslocators 126 described above with respect to FIG. 1.

In some embodiments, near-eye display 300 may also include ahigh-resolution camera 340. Camera 340 may capture images of thephysical environment in the field of view. The captured images may beprocessed, for example, by a virtual reality engine (e.g., artificialreality engine 116 of FIG. 1) to add virtual objects to the capturedimages or modify physical objects in the captured images, and theprocessed images may be displayed to the user by display 310 for AR orMR applications.

FIG. 4 illustrates an example of an optical see-through augmentedreality system 400 using a waveguide display according to certainembodiments. Augmented reality system 400 may include a projector 410and a combiner 415. Projector 410 may include a light source or imagesource 412 and projector optics 414. In some embodiments, image source412 may include a plurality of pixels that displays virtual objects,such as an LCD display panel or an LED display panel. In someembodiments, image source 412 may include a light source that generatescoherent or partially coherent light. For example, image source 412 mayinclude a laser diode, a vertical cavity surface emitting laser, and/ora light emitting diode. In some embodiments, image source 412 mayinclude a plurality of light sources each emitting a monochromatic imagelight corresponding to a primary color (e.g., red, green, or blue). Insome embodiments, image source 412 may include an optical patterngenerator, such as a spatial light modulator. Projector optics 414 mayinclude one or more optical components that can condition the light fromimage source 412, such as expanding, collimating, scanning, orprojecting light from image source 412 to combiner 415. The one or moreoptical components may include, for example, one or more lenses, liquidlenses, mirrors, apertures, and/or gratings. In some embodiments,projector optics 414 may include a liquid lens (e.g., a liquid crystallens) with a plurality of electrodes that allows scanning of the lightfrom image source 412.

Combiner 415 may include an input coupler 430 for coupling light fromprojector 410 into a substrate 420 of combiner 415. Input coupler 430may include a volume holographic grating, a diffractive optical elements(DOE) (e.g., a surface-relief grating), or a refractive coupler (e.g., awedge or a prism). Input coupler 430 may have a coupling efficiency ofgreater than 30%, 50%, 75%, 90%, or higher for visible light. As usedherein, visible light may refer to light with a wavelength between about380 nm to about 750 nm. Light coupled into substrate 420 may propagatewithin substrate 420 through, for example, total internal reflection(TIR). Substrate 420 may be in the form of a lens of a pair ofeyeglasses. Substrate 420 may have a flat or a curved surface, and mayinclude one or more types of dielectric materials, such as glass,quartz, plastic, polymer, poly(methyl methacrylate) (PMMA), crystal, orceramic. A thickness of the substrate may range from, for example, lessthan about 1 mm to about 10 mm or more. Substrate 420 may be transparentto visible light. A material may be “transparent” to a light beam if thelight beam can pass through the material with a high transmission rate,such as larger than 50%, 40%, 75%, 80%, 90%, 95%, or higher, where asmall portion of the light beam (e.g., less than 50%, 40%, 25%, 20%,10%, 5%, or less) may be scattered, reflected, or absorbed by thematerial. The transmission rate (i.e., transmissivity) may berepresented by either a photopically weighted or an unweighted averagetransmission rate over a range of wavelengths, or the lowesttransmission rate over a range of wavelengths, such as the visiblewavelength range.

Substrate 420 may include or may be coupled to a plurality of outputcouplers 440 configured to extract at least a portion of the lightguided by and propagating within substrate 420 from substrate 420, anddirect extracted light 460 to an eye 490 of the user of augmentedreality system 400. As with input coupler 430, output couplers 440 mayinclude grating couplers (e.g., volume holographic gratings orsurface-relief gratings), other DOEs, prisms, etc. Output couplers 440may have different coupling (e.g., diffraction) efficiencies atdifferent locations. Substrate 420 may also allow light 450 fromenvironment in front of combiner 415 to pass through with little or noloss. Output couplers 440 may also allow light 450 to pass through withlittle loss. For example, in some implementations, output couplers 440may have a low diffraction efficiency for light 450, such that light 450may be refracted or otherwise pass through output couplers 440 withlittle loss, and thus may have a higher intensity than extracted light460. In some implementations, output couplers 440 may have a highdiffraction efficiency for light 450 and may diffract light 450 tocertain desired directions (i.e., diffraction angles) with little loss.As a result, the user may be able to view combined images of theenvironment in front of combiner 415 and virtual objects projected byprojector 410.

FIG. 5 illustrates propagations of incident display light 540 andexternal light 530 in an example of a waveguide display 500 including awaveguide 510 and a grating coupler 520. Waveguide display 500 mayinclude, for example, combiner 415 of FIG. 4. Waveguide 510 may be aflat or curved transparent substrate with a refractive index n₂ greaterthan the free space refractive index n₁ (i.e., 1.0). Grating coupler 520may include, for example, a Bragg grating or a surface-relief grating.

Incident display light 540 may be coupled into waveguide 510 by, forexample, input coupler 430 of FIG. 4 or other couplers (e.g., a prism orslanted surface) described above. Incident display light 540 maypropagate within waveguide 510 through, for example, total internalreflection. When incident display light 540 reaches grating coupler 520,incident display light 540 may be diffracted by grating coupler 520into, for example, a 0^(th) order diffraction (i.e., reflection) light542 and a −1st order diffraction light 544. The 0^(th) order diffractionmay continue to propagate within waveguide 510, and may be reflected bythe bottom surface of waveguide 510 towards grating coupler 520 at adifferent location. The −1st order diffraction light 544 may be coupled(e.g., refracted) out of waveguide 510 towards the user's eye, because atotal internal reflection condition may not be met at the bottom surfaceof waveguide 510 due to the diffraction angle of the −1^(st) orderdiffraction light 544.

External light 530 may also be diffracted by grating coupler 520 into,for example, a 0^(th) order diffraction light 532 or a −1st orderdiffraction light 534. The 0^(th) order diffraction light 532 or the−1st order diffraction light 534 may be refracted out of waveguide 510towards the user's eye. Thus, grating coupler 520 may act as an inputcoupler for coupling external light 530 into waveguide 510, and may alsoact as an output coupler for coupling incident display light 540 out ofwaveguide 510. As such, grating coupler 520 may act as a combiner forcombining external light 530 and incident display light 540 and send thecombined light to the user's eye.

In order to diffract light in a desired direction towards the user's eyeand to achieve a desired diffraction efficiency for certain diffractionorders, grating coupler 520 may include a blazed or slanted grating,such as a slanted Bragg grating or surface-relief grating, where thegrating ridges and grooves may be tilted relative to the surface normalof grating coupler 520 or waveguide 510.

FIG. 6 illustrates an example slanted grating 620 in an examplewaveguide display 600 according to certain embodiments. Slanted grating620 may be an example of output couplers 440 or grating coupler 520.Waveguide display 600 may include slanted grating 620 on a waveguide610, such as substrate 420 or waveguide 510. Slanted grating 620 may actas a grating coupler for coupling light into or out of waveguide 610. Insome embodiments, slanted grating 620 may include a periodic structurewith a period p. For example, slanted grating 620 may include aplurality of ridges 622 and grooves 624 between ridges 622. Each periodof slanted grating 620 may include a ridge 622 and a groove 624, whichmay be an air gap or a region filled with a material with a refractiveindex n_(g2). The ratio between the width w of a ridge 622 and thegrating period p may be referred to as duty cycle. Slanted grating 620may have a duty cycle ranging, for example, from about 10% to about 90%or greater. In some embodiments, the duty cycle may vary from period toperiod. In some embodiments, the period p of the slanted grating mayvary from one area to another on slanted grating 620, or may vary fromone period to another (i.e., chirped) on slanted grating 620.

Ridges 622 may be made of a material with a refractive index of n_(g1),such as silicon containing materials (e.g., SiO₂, Si₃N₄, SiC,SiO_(x)N_(y), or amorphous silicon), organic materials (e.g., spin oncarbon (SOC) or amorphous carbon layer (ACL) or diamond like carbon(DLC)), or inorganic metal oxide layers (e.g., TiO_(x), AlO_(x),TaO_(x), HfO_(x), etc.). Each ridge 622 may include a leading edge 630with a slant angle α and a trailing edge 640 with a slant angle β. Insome embodiments, leading edge 630 and training edge 640 of each ridge622 may be parallel to each other. In other words, slant angle α isapproximately equal to slant angle β. In some embodiments, slant angle αmay be different from slant angle β. In some embodiments, slant angle αmay be approximately equal to slant angle β. For example, the differencebetween slant angle α and slant angle θ may be less than 20%, 10%, 5%,1%, or less. In some embodiments, slant angle α and slant angle θ mayrange from, for example, about 30° or less to about 70% or more.

In some implementations, grooves 624 between the ridges 622 may beover-coated or filled with a material having a refractive index n_(g2)higher or lower than the refractive index of the material of ridges 622.For example, in some embodiments, a high refractive index material, suchas Hafnia, Titania, Tantalum oxide, Tungsten oxide, Zirconium oxide,Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, and a highrefractive index polymer, may be used to fill grooves 624. In someembodiments, a low refractive index material, such as silicon oxide,alumina, porous silica, or fluorinated low index monomer (or polymer),may be used to fill grooves 624. As a result, the difference between therefractive index of the ridges and the refractive index of the groovesmay be greater than 0.1, 0.2, 0.3, 0.5, 1.0, or higher.

The slanted gratings described above and other surface-relief gratings(e.g., gratings used for eye-tracking) may be fabricated using manydifferent nanofabrication techniques. The nanofabrication techniquesgenerally include a patterning process and a post-patterning (e.g.,over-coating) process. The patterning process may be used to formslanted ridges of the slanted grating. There may be many differentnanofabrication techniques for forming the slanted ridges. For example,in some implementations, the slanted grating may be fabricated usinglithography techniques including slanted etching. In someimplementations, the slanted grating may be fabricated using nanoimprintlithography (NIL) molding techniques, where a master mold includingslanted structures may be fabricated using, for example, slanted etchingtechniques, and may then be used to mold slanted gratings or differentgenerations of soft stamps for nanoimprinting. The post-patterningprocess may be used to over-coat the slanted ridges and/or to fill thegaps between the slanted ridges with a material having a differentrefractive index than the slanted ridges. The post-patterning processmay be independent from the patterning process. Thus, a samepost-patterning process may be used on slanted gratings fabricated usingany pattering technique.

Techniques and processes for fabricating slanted gratings describedbelow are for illustration purposes only and are not intended to belimiting. A person skilled in the art would understand that variousmodifications may be made to the techniques described below. Forexample, in some implementations, some operations described below may beomitted. In some implementations, additional operations may be performedto fabricate the slanted grating. Techniques disclosed herein may alsobe used to fabricate other slanted structures on various materials.

FIGS. 7A-7C illustrate an example of a process for fabricating a slantedsurface-relief grating by slanted etching according to certainembodiments. FIG. 7A shows a structure 700 after a lithography process,such as a photolithography or electron beam lithography process.Structure 700 may include a substrate 710 that may be used as thewaveguide of a waveguide display described above, such as a glass orquartz substrate. In some embodiments, structure 700 may also include alayer of grating material 720, such as Si₃N₄, SiO₂, titanium oxide,alumina, and the like. Substrate 710 may have a refractive index n_(wg),and the layer of grating material 720 may have a refractive indexn_(g1). In some embodiments, the layer of grating material 720 may be apart of substrate 710. A mask layer 730 with a desired pattern may beformed on top of the layer of grating material 720. Mask layer 730 mayinclude, for example, a photoresist material, a metal (e.g., copper,chrome, titanium, aluminum, or molybdenum), an intermetallic compound(e.g., MoSiON), or an organic material (e.g., polymer). Mask layer 730may be referred to as a hard mask layer. Mask layer 730 may be formedby, for example, an optical projection (using a photomask) or electronbeam lithography process, a nanoimprint lithography process, amulti-beam interference process, and the like.

FIG. 7B shows a structure 740 after a slanted etching process, such as adry etching process (e.g., RIE, inductively coupled plasma (ICP)etching, deep silicon etching (DSE), IBE, or variations of IBE). Theslanted etching process may include one or more sub-steps. The slantedetching may be performed by, for example, rotating structure 700 withrespect to the direction of the etching beam based on the desired slantangle and etching the layer of grating material 720 by the etching beam.After the etching, a slanted grating 750 may be formed in the layer ofgrating material 720.

FIG. 7C shows a structure 770 after mask layer 730 is removed. Structure770 may include substrate 710, the layer of grating material 720, andslanted grating 750. Slanted grating 750 may include a plurality ofridges 752 and grooves 754. Techniques such as plasma or wet etching maybe used to strip mask layer 730 with appropriate chemistry. In someimplementations, mask layer 730 may not be removed and may be used aspart of the slanted grating. The width of each ridge 752 may be referredto as the line width. In some embodiments, the minimum feature size ofmask layer 730 or the minimum line width of ridges 752 (which may bereferred to as the critical dimension (CD) of a process) that can bereliably manufactured using the process may be limited due to, forexample, the wavelength of the light used in the photolithography, thenumerical aperture of the photolithography system, and otherprocess-related factors (which may be referred to as k₁ factor).

Subsequently, in some implementations, a post-patterning (e.g.,over-coating) process may be performed to over-coat slanted grating 750with a material having a refractive index higher or lower than thematerial of ridges 752. For example, as described above, in someembodiments, a high refractive index material, such as Hafnia, Titania,Tungsten oxide, Zirconium oxide, Gallium sulfide, Gallium nitride,Gallium phosphide, silicon, and a high refractive index polymer, may beused for the over-coating. In some embodiments, a low refractive indexmaterial, such as silicon oxide, alumina, porous silica, or fluorinatedlow index monomer (or polymer), may be used for the over-coating. As aresult, the difference between the refractive index of ridges 752 andthe refractive index of the over-coating material in grooves 754 may begreater than 0.1, 0.2, 0.3, 0.5, 1.0, or higher.

FIGS. 8A and 8B illustrate an example process for fabricating a slantedsurface-relief grating by direct molding according to certainembodiments. In FIG. 8A, a waveguide 810 may be coated with a NIL resinlayer 820. NIL resin layer 820 may include, for example, abutyl-acrylate-based resin doped with a sol-gel precursor (e.g.,titanium butoxide), a monomer containing a reactive functional group forsubsequent infusion processes (such as acrylic acid), and/or highrefractive index nanoparticles (e.g., TiO₂, GaP, HfO₂, GaAs, etc.). Insome embodiments, NIL resin layer 820 may include polydimethylsiloxane(PDMS) or another silicone elastomer or silicon-based organic polymer.NIL resin layer 820 may be deposited on waveguide 810 by, for example,spin-coating, lamination, or ink injection. A NIL mold 830 with slantedridges 832 may be pressed against NIL resin layer 820 and waveguide 810for molding a slanted grating in NIL resin layer 820. NIL resin layer820 may be cured subsequently (e.g., cross-linked) using heat and/orultraviolet (UV) light.

FIG. 8B shows the demolding process, during which NIL mold 830 isdetached from NIL resin layer 820 and waveguide 810. As shown in FIG.8B, after NIL mold 830 is detached from NIL resin layer 820 andwaveguide 810, a slanted grating 822 that is complementary to slantedridges 832 in NIL mold 830 may be formed in NIL resin layer 820 onwaveguide 810.

In some embodiments, a master NIL mold (e.g., a hard mold including arigid material, such as Si, SiO₂, Si₃N₄, or a metal) may be fabricatedfirst using, for example, slanted etching, micromachining, or 3Dprinting. A soft stamp may be fabricated using the master NIL mold, andthe soft stamp may then be used as the working stamp to fabricate theslanted grating or may be used to fabricate a next generation softstamp. In such a process, the slanted grating structure in the masterNIL mold may be similar to the slanted grating of the grating couplerfor the waveguide display, and the slanted grating structure on the softstamp may be complementary to the slanted grating structure in themaster NIL mold and the slanted grating of the grating coupler for thewaveguide display. Compared with a hard stamp or hard mold, a soft stampmay offer more flexibility during the molding and demolding processes.

FIGS. 9A-9D illustrate an example process 900 for fabricating a softstamp used for making a slanted surface-relief grating according tocertain embodiments. FIG. 9A shows a master mold 910 (e.g., a hard moldor hard stamp). Master mold 910 may include a rigid material, such as asemiconductor substrate (e.g., Si or GaAs), an oxide (e.g., SiO₂, Si₃N₄,TiO_(x), AlO_(x), TaO_(x), or HfO_(x)), or a metal plate. Master mold910 may be fabricated using, for example, a slanted etching processusing reactive ion beams or chemically assisted reactive ion beams, amicromachining process, or a 3D printing process. As shown in FIG. 9A,master mold 910 may include a slanted grating 920 that may in turninclude a plurality of slanted ridges 922 with gaps 924 between slantedridges 922.

FIG. 9B illustrates master mold 910 coated with a soft stamp materiallayer 930. Soft stamp material layer 930 may include, for example, aresin material or a curable polymer material. In some embodiments, softstamp material layer 930 may include polydimethylsiloxane (PDMS) oranother silicone elastomer or silicon-based organic polymer. In someembodiments, soft stamp material layer 930 may include ethylenetetrafluoroethylene (ETFE), perfluoropolyether (PFPE), or otherfluorinated polymer materials. In some embodiments, soft stamp materiallayer 930 may be coated on master mold 910 by, for example, spin-coatingor ink injection.

FIG. 9C illustrates a lamination process for laminating a soft stampfoil 940 onto soft stamp material layer 930. A roller 950 may be used topress soft stamp foil 940 against soft stamp material layer 930. Thelamination process may also be a planarization process to make thethickness of soft stamp material layer 930 substantially uniform. Afterthe lamination process, soft stamp foil 940 may be tightly or securelyattached to soft stamp material layer 930.

FIG. 9D illustrates a delamination process, where a soft stamp includingsoft stamp foil 940 and attached soft stamp material layer 930 isdetached from master mold 910. Soft stamp material layer 930 may includea slanted grating structure that is complementary to the slanted gratingstructure on master mold 910. Because the flexibility of soft stamp foil940 and attached soft stamp material layer 930, the delamination processmay be relatively easy compared with a demolding process using a hardstamp or mold. In some embodiments, a roller (e.g., roller 950) may beused in the delamination process to ensure a constant or controlleddelamination speed. In some embodiments, roller 950 may not be usedduring the delamination. In some implementations, an anti-sticking layermay be formed on master mold 910 before soft stamp material layer 930 iscoated on master mold 910. The anti-sticking layer may also facilitatethe delamination process. After the delamination of the soft stamp frommaster mold 910, the soft stamp may be used to mold the slanted gratingon a waveguide of a waveguide display.

FIGS. 10A-10D illustrate an example process 1000 for fabricating aslanted surface-relief grating using a soft stamp according to certainembodiments. FIG. 10A shows a waveguide 1010 coated with an imprintresin layer 1020. Imprint resin layer 1020 may include, for example, abutyl-acrylate based resin doped with a sol-gel precursor (e.g.,titanium butoxide), a monomer containing a reactive functional group forsubsequent infusion processes (such as acrylic acid), and/or highrefractive index nanoparticles (e.g., TiO₂, GaP, HfO₂, GaAs, etc.). Insome embodiments, imprint resin layer 1020 may includepolydimethylsiloxane (PDMS) or another silicone elastomer orsilicon-based organic polymer. In some embodiments, imprint resin layer1020 may include ethylene tetrafluoroethylene (ETFE), perfluoropolyether(PFPE), or other fluorinated polymer materials. Imprint resin layer 1020may be deposited on waveguide 1010 by, for example, spin-coating,lamination, or ink injection. A soft stamp 1030 including slanted ridges1032 attached to a soft stamp foil 1040 may be used for the imprint.

FIG. 10B shows the lamination of soft stamp 1030 onto imprint resinlayer 1020. Soft stamp 1030 may be pressed against imprint resin layer1020 and waveguide 1010 using a roller 1050, such that slanted ridges1032 may be pressed into imprint resin layer 1020. Imprint resin layer1020 may be cured subsequently. For example, imprint resin layer 1020may be cross-linked using heat and/or UV light.

FIG. 10C shows the delamination of soft stamp 1030 from imprint resinlayer 1020. The delamination may be performed by lifting soft stamp foil1040 to detach slanted ridges 1032 of soft stamp 1030 from imprint resinlayer 1020. Imprint resin layer 1020 may now include a slanted grating1022, which may be used as the grating coupler or may be over-coated toform the grating coupler for the waveguide display. As described above,because of the flexibility of soft stamp 1030, the delamination processmay be relatively easy compared with a demolding process using a hardstamp or mold. In some embodiments, a roller (e.g., roller 1050) may beused in the delamination process to ensure a constant or controlleddelamination speed. In some embodiments, roller 1050 may not be usedduring the delamination.

FIG. 10D shows an example imprinted slanted grating 1022 formed onwaveguide 1010 using soft stamp 1030. As described above, slantedgrating 1022 may include ridges and gaps between the ridges and thus maybe over-coated with a material having a refractive index different fromimprint resin layer 1020 to fill the gaps and form the grating couplerfor the waveguide display.

In various embodiments, the period of the slanted grating may vary fromone area to another on slanted grating 1022, or may vary from one periodto another (i.e., chirped) on slanted grating 1022. Slanted grating 1022may have a duty cycle ranging, for example, from about 10% to about 90%or greater. In some embodiments, the duty cycle may vary from period toperiod. In some embodiments, the depth or height of the ridges ofslanted grating 1022 may be greater than 50 nm, 100 nm, 200 nm, 300 nm,400 nm, or higher. The slant angles of the leading edges of the ridgesof slanted grating 1022 and the slant angles of the trailing edges ofthe ridges of slanted grating 1022 may be greater than 30°, 45°, 60°, orhigher. In some embodiments, the leading edge and training edge of eachridge of slanted grating 1022 may be parallel to each other. In someembodiments, the difference between the slant angle of the leading edgeof a ridge of slanted grating 1022 and the slant angle of the trailingedge of the ridge of slanted grating 1022 may be less than 20%, 10%, 5%,1%, or less.

FIG. 11 is a simplified flow chart 1100 illustrating example methods offabricating a slanted surface-relief grating using nanoimprintlithography according to certain embodiments. As described above,different generations of NIL stamps may be made and used as the workingstamp to mold the slanted gratings. For example, in some embodiments, amaster mold (i.e., generation 0 mold, which may be a hard mold) may beused as the working stamp to mold the slanted grating directly. In someembodiments, a hybrid stamp (e.g., a generation 1 hybrid mold or stamp)may be fabricated using the master mold and may be used as the workingstamp for nanoimprinting. In some embodiments, a generation 2 hybridmold (or stamp) may be made from the generation 1 mold, and may be usedas the working stamp for the nanoimprinting. In some embodiments, ageneration 3 mold, a generation 4 mold, and so on, may be made and usedas the working stamp.

At block 1110, a master mold with a slanted structure may be fabricatedusing, for example, a slanted etching process that uses reactive ionbeams or chemically-assisted reactive ion beams, a micromachiningprocess, or a 3D printing process. The master mold may be referred to asthe generation 0 (or Gen 0) mold. The master mold may include quartz,fused silica, silicon, other metal-oxides, or plastic compounds. Theslanted structure of the master mold may be referred to as having apositive (+) tone. The master mold may be used as a working stamp formolding the slanted grating directly (i.e., hard NIL) at block 1120. Asdescribed above, when the master mold is used as the working stamp, theslanted structure of the master mold may be complementary to the desiredslanted grating. Alternatively, the master mold may be used to make ahybrid stamp as the working stamp for molding the slanted grating. Theslanted structure of the hybrid stamp may be similar to the desiredslanted grating or may be complementary to the desired slanted grating,depending on the generation of the hybrid stamp.

At block 1120, a slanted grating may be molded in, for example, a resinlayer using the master mold as described above with respect to, forexample, FIGS. 9A and 9B. The resin layer may be coated on a waveguidesubstrate, and may include, for example, a butyl-acrylate based resindoped with a resin comprising a sol-gel precursor (e.g., titaniumbutoxide), a monomer containing a reactive functional group forsubsequent infusion processes (such as acrylic acid), and/or highrefractive index nanoparticles (e.g., TiO₂, GaP, HfO₂, GaAs, etc.). Themaster mold may be pressed against the resin layer. The resin layer maythen be cured to fix the structure formed within the resin layer by themaster mold. The master mold may be detached from the resin layer toform a slanted grating within the resin layer. The slanted gratingwithin the resin layer may have a negative (−) tone compared with theslanted structure of the master mold.

Alternatively, at block 1130, a hybrid stamp (e.g., a hard stamp, a softstamp, or a hard-soft stamp) with a slanted structure may be fabricatedusing the master mold as described above with respect to, for example,FIGS. 9A-9D or the process described with respect to, for example, FIGS.10A-10D. For example, the process of fabricating the hybrid stamp mayinclude coating the master mold with a soft stamp material, such as aresin material described above. A soft stamp foil may then be laminatedon the soft stamp material, for example, using a roller. The soft stampfoil and the attached soft stamp material may be securely attached toeach other and may be detached from the master mold to form the softstamp. The hybrid stamp fabricated at block 1130 may be referred to as ageneration 1 (or Gen 1) stamp. The slanted grating within the Gen 1stamp may have a negative (−) tone compared with the slanted structureof the master mold.

At block 1140, a slanted surface-relief grating may be imprinted usingthe Gen 1 stamp as described above with respect to, for example, FIGS.9A-9D. For example, a waveguide substrate may be coated with an imprintresin layer. The Gen 1 stamp may be laminated on the imprint resin layerusing, for example, a roller. After the imprint resin layer is cured,the Gen 1 stamp may be delaminated from the imprint resin layer to forma slanted grating within the imprint resin layer. The slanted gratingwithin the imprint resin layer may have a positive tone.

Alternatively, in some embodiments, at block 1150, a second generationhybrid stamp (Gen 2 stamp) may be fabricated using the Gen 1 stamp usinga process similar to the process for fabricating the Gen 1 stamp asdescribed above with respect to, for example, FIGS. 9A-9D. The slantedstructure within the Gen 2 stamp may have a positive tone.

At block 1160, a slanted surface-relief grating may be imprinted usingthe Gen 2 stamp as described above with respect to, for example, FIGS.9A-9D. For example, a waveguide substrate may be coated with an imprintresin layer. The Gen 2 stamp may be laminated on the imprint resin layerusing, for example, a roller. After the imprint resin layer is cured,the Gen 2 stamp may be delaminated from the imprint resin layer to forma slanted grating within the imprint resin layer. The slanted gratingwithin the imprint resin layer may have a negative tone.

Alternatively, in some embodiments, at block 1170, a second generation(Gen 2) daughter mold may be fabricated using the Gen 1 stamp using aprocess similar to the process for fabricating the Gen 1 stamp asdescribed above with respect to, for example, FIGS. 9A-9D. The slantedstructure within the Gen 2 daughter mold may have a positive tone.

At block 1180, a third generation hybrid stamp (Gen 3 stamp) may befabricated using the Gen 2 daughter mold using a process similar to theprocess for fabricating the Gen 1 stamp or the Gen 2 daughter mold asdescribed above with respect to, for example, FIGS. 9A-9D. The slantedstructure within the Gen 3 stamp may have a negative tone.

At block 1190, a slanted surface-relief grating may be imprinted usingthe Gen 3 stamp as described above with respect to, for example, FIGS.10A-10D. For example, a waveguide substrate may be coated with animprint resin layer. The Gen 3 stamp may be laminated on the imprintresin layer using, for example, a roller. After the imprint resin layeris cured, the Gen 3 stamp may be delaminated from the imprint resinlayer to form a slanted grating within the imprint resin layer. Theslanted grating within the imprint resin layer may have a positive tone.

Even though not shown in FIG. 11, in some embodiments, a fourthgeneration hybrid stamp, a fifth generation hybrid stamp, and so on, maybe fabricated using a similar process, and may be used as the workingstamp for imprinting the slanted grating. In some implementations, thesurface of any of the master mold, gen 1 stamp, Gen 2 stamp, and Gen 3stamp may be coated or plated prior to imprinting to reduce wearing ofthe mold, improve product quality, and reduce manufacturing cost. Forexample, in some implementations, an anti-sticking layer may be coatedon the mold before the molding (or imprinting) process.

Optionally, at block 1195, the slanted grating may be over-coated with amaterial having a refractive index different from the slanted grating(e.g., the imprint resin layer). For example, in some embodiments, ahigh refractive index material, such as Hafnia, Titania, Tungsten oxide,Zirconium oxide, Gallium sulfide, Gallium nitride, Gallium phosphide,silicon, or a high refractive index polymer, may be used to over-coatthe slanted grating and fill the gaps between the slanted gratingridges. In some embodiments, a low refractive index material, such assilicon oxide, magnesium fluoride, porous silica, or fluorinated lowindex monomer (or polymer), and the like, may be used to over-coat theslanted grating and fill the gaps between the slanted grating ridges.

In some embodiments, an etching system, such as an ion beam etching(IBE) system, may be used to etch a surface-relief structure, such as aslanted surface-relief grating described above or a master mold forimprinting a surface-relief grating for a waveguide-based near-eyedisplay system, as described above with respect to FIGS. 7A-7C.

FIG. 12 illustrates an example of an ion beam etching (IBE) system 1200for etching a slanted surface-relief structure according to certainembodiments. Ion beam etching generally uses a highly collimated andhighly directional ion beam to physically mill materials from asubstrate that is mounted on a rotation stage with an adjustablerotation angle. IBE system 1200 may include an ion source generator1210. Ion source generator 1210 may include an inert gas inlet 1220 forreceiving an inert gas, such as argon gas, into a chamber of ion sourcegenerator 1210. A plasma may be generated in ion source generator 1210via a radio frequency (RF) inductively coupled plasma (ICP) generator1230, where highly energetic electrons may ionize neutrals of theinjected inert gas (e.g., Ar) through collisions with the neutrals. Ahigh density plasma 1222 may be generated within ion source generator1210 by the impact ionization. High density plasma 1222 may beconsidered as a sea of neutrals with positive ions 1224 and negativeelectrons in charge equilibrium.

IBE system 1200 may also include one or more aligned collimator gridsfor extracting a collimated ion beam 1262 from high density plasma 1222that is formed within ion source generator 1210. The aligned collimatorgrids may be implemented in various ways. For example, as shown in FIG.12, the aligned collimator grids may include an extraction grid 1240that may contact high density plasma 1222 and control its potential, andan acceleration grid 1250 that may be driven by an adjustable negativehigh voltage supply for accelerating the extracted ions. A beamneutralizer 1260 may be disposed near the aligned collimator grids andmay emit an electron beam into collimated ion beam 1262 to achieve a netneutral charge flux associated with collimated ion beam 1262 in order toprevent the buildup of positive charges on the structure to be etched.

The highly directional collimated ion beam 1262 may physically millmaterials from a material layer 1280 to be etched, such as, for example,a semiconductor wafer, a glass substrate, a Si₃N₄ material layer, atitanium oxide layer, an alumina layer, and the like. Material layer1280 may be partially covered by a mask 1282, which may be formed onmaterial layer 1280 by, for example, a photolithography process. Mask1282 may include, for example, a photoresist material, a metal (e.g.,copper, chrome, aluminum, or molybdenum), an intermetallic compound(e.g., MoSi₂), or a polymer. In some embodiments, a shutter 1290 (orblade) may be used to control the etch time and/or the etch region.Material layer 1280 may be mounted on a rotation stage 1270 that can berotated to modify the angle of material layer 1280 with respect to thehighly directional collimated ion beam 1262. The ability to modify theangle of material layer 1280 may allow for the creation of tailoredsidewall profiles (e.g., slant angle) with minimal sputteredre-deposition on mask 1282.

In some embodiments, a chemically assisted ion beam etching (CAIBE)system may be used for fabricating a slanted surface-relief structure.In the chemically assisted ion beam etching, reactive species, such as areactive gas (e.g., CF₄, CHF₃, N₂, O₂, SF₆, Cl₂, BCl₃, HBr, etc.) may beintroduced into the process independent of the ion beam. Thus, thematerial layer to be etched may be etched both physically andchemically.

In some embodiments, a reactive ion beam etching (RIBE) system may beused for fabricating a slanted surface-relief structure. The reactiveion beam etching system may be similar to IBE system 1200, except that areactive gas (e.g., CF₄, CHF₃, N₂, O₂, SF₆, etc.) may also be injectedinto the ion source generator to form a reactive ion beam that can bothphysically and chemically etch the material layer to be etched.

As described above, to selectively couple display light and/or ambientlight into and/or out of the waveguide and into user's eyes, improve thefield of view, increase brightness or power efficiency, reduce displayartifacts (e.g., rainbow artifacts), and improve other performances of awaveguide display, a slanted surface-relief grating having a large rangeof grating duty cycles (e.g., from about 0.3 to about 0.9), large slantangles (e.g., greater than 30°, 45°, 60°, or larger), small periods(e.g., less than a micron), and high depths (e.g., greater than 100 nm,200 nm, or 300 nm) may be desired. However, it may be challenging tomore efficiently and more accurately manufacture a slanted grating thathas a large depth, a large slanted angle, and a wide range of dutycycles (in particular, large duty cycles) on a substrate.

As also described above, the slanted surface-relief structures may befabricated using many different nanofabrication techniques, such aslithography and etching techniques or NIL molding techniques. The NILmolding techniques may use a master mold to make different generationsof stamps for the NIL molding as described above with respect to FIGS.8A-11. The nanostructure in the master mold may be etched using, forexample, etching techniques described above. However, when the slantedsurface-relief structure to be fabricated has a large slant angle (e.g.,greater than 30°, 45°, or 60°), a small grating period (e.g., less thana few microns, 1 μm, 500 nm, 200 nm, 100 nm, or lower), a high depth(e.g., >100 nm), a high aspect ratio (e.g., 3:1, 5:1, 10:1, or larger),and/or a large or small duty cycle (e.g., below 30% or greater than70%), it may be difficult to etch or imprint a slanted surface-reliefstructure due to certain properties of the etching or nanoimprintingprocess. In a nanoimprint process, when the desired slantedsurface-relief structure has both a high grating depth and a large dutycycle, cracking or breaking at least some grating ridges of the mold,stamp, or the imprinted slanted surface-relief structure may occurduring, for example, the demolding process, due to the small featuresizes of the grating ridges of the mold, stamp, or the imprinted slantedsurface-relief structure. In a dry etching process, the etch rate may belower when the duty cycle is large and/or when the period of the gratingis small (e.g., less than 1 μm, 500 nm, 200 nm, or 100 nm) such that thetrenches to be etched may have high aspect ratios, while the etch ratemay be higher when the duty cycle is small and/or when the gratingperiod is large. Thus, the etch depth by an etching process maycorrelate with the duty cycle and/or the grating period, and may not bea parameter that is independent from the duty cycle and/or the gratingperiod and thus can be independently selected or tuned. In addition, theetch rate for etching a grating with a large duty cycle may be very low.

FIG. 13A illustrates an example of etching a slanted grating 1320 usinga fabrication process according to certain embodiments. The fabricationprocess shown in FIG. 13 may be a physical or physical/chemical etchingprocess, where ions or other particles may be used to bombard asubstrate 1310 and/or react with the material in substrate 1310. A mask1340 may be used to block the particles in regions not to be etched,which may form the ridges of slanted grating 1320.

For a deep straight or slanted grating with a small duty cycle or alarge grating period, the grating grooves may be relatively wide or mayhave a lower aspect ratio (depth divided by width) and thus the residuesfrom the etching may be relatively easy to remove, and thus the etchrate may be higher. However, for a deep grating with a large duty cycleor a small period where the grating grooves may have higher aspectratios, even if the grating may be a straight grating rather than aslanted grating, the residues from the etching may be relativelydifficult to remove from the etched regions, and thus the etch rate maybe relatively lower. In addition, depletion of ions may occur when ionsare captured by the sidewalls due to the angular distribution ofincoming ions into the trench opening and the electrostatic fields inthe trench, which may also reduce the etch rate. Thus, under a sameetching condition (e.g., same ion beam dosage and etch time), regions ofthe grating having a smaller duty cycle may be etched faster and deeperthan regions of the grating having a larger duty cycle. As such, fornanostructures with trenches of different dimensions and/or aspectratios coexisting on a same substrate, the etched depths may bedifferent for different trenches after a same etching process.

As also shown in FIG. 13A, on one side (e.g., trailing edge) of a ridgeof slanted grating 1320, incoming particles may hit the trailing edge asshown by the line 1330, and a molecule or atom 1322 at the surface ofthe grating ridge may be dislocated from the grating ridge due to theimpact by the incoming particles. Molecule or atom 1322 of the gratingmaterial may move in a direction such that it can be easily removed fromthe grating ridge region as shown in FIG. 13A. Thus, the dislocatedmolecules or atoms would not accumulate at the bottom of the grooves ofthe slanted grating. On the other side (e.g., the leading edge) of theridge of the slanted grating, incoming particles may hit the leadingedge as shown by the line 1332, and a molecule or atom 1324 at thesurface of the grating ridge may be dislocated from the grating ridgedue to the impact by the incoming particles. However, due to thedirection of the incoming particles, molecule or atom 1324 of thegrating material may be pushed further into the grating ridge or may beremove from the grating ridge at a direction such that molecule or atom1324 may accumulate at the bottom of the grooves of the slanted grating.As such, the etch rate at the leading edge of the grating ridges may belower and/or the materials etched from the leading edges of the gratingridges may accumulate at the bottom of the grooves of the slantedgrating, which may cause the slant angle or depth of the leading edge tobe different from the slant angle or depth of the trailing edge.

FIG. 13B illustrates an example of a slanted grating 1350 fabricatedusing an etching process. As shown, in section 1352, the grating dutycycle may be small and thus the grating grooves may be wider. Therefore,the etch depth in slanted grating 1350 may be higher, such as about 200nm as shown in the example. In contrast, in section 1354 where thegrating duty cycle may be large and thus the grating grooves may benarrower, the etch depth in slanted grating 1350 may be lower, such asabout 75 nm as shown in FIG. 13B. This effect may be referred to as theRIE lag effect, where smaller trenches may be etched at lower rates thanlarger trenches.

FIG. 14 illustrates an example of an RIE lag curve 1410, whichrepresents the relationship between duty cycles and etch depths ofstructures etched using RIE. Specifically, the horizontal axisrepresents the duty cycle values, which range from 0% to 100%. Asdiscussed above, for a structure having ridges and trenches, the dutycycle refers to the ratio between the width of a ridge and the combinedwidth of the ridge and the adjacent trench (i.e., a period). Thevertical axis represents the relative etch depth that can be achievedfor each respective duty cycle using a same RIE process after a sameetch duration. It should be noted that RIE lag curve 1410 shown in FIG.14 is for illustration purposes only and may only illustrate a generaltrend. Depending on the materials to be etched, the etch system used,the etchants used, and/or the etching conditions, the actual RIE lagcurve may vary from one etching condition to another etching condition.

As shown in FIG. 14, as the duty cycle increases (i.e., the width of theridge increases and/or the width of the trench decreases), for a givenetch duration, the etch depth that can be achieved by an ME processgradually decreases. This is because there are abundant ions for theetch reaction to occur regardless of the duty cycle, but the ion meanfree path may be shorter inside small trenches, which may lower theeffectiveness of the etching. Also, the etching by-products may not beefficiently transported out of the etched trenches when the duty cycleis relatively large. The lag in the by-product transport to the trenchopening may result in a lower etch depth when the duty cycle is larger.

In the example shown in FIG. 14, when the duty cycle changes from, forexample, below about 50% to about 90% or higher, the etch depth canreduce significantly. It should be noted that although FIG. 14illustrates that when the duty cycle is about 50% or below, the effectof duty cycle on the etch depth may be relatively small in someembodiments, in some other embodiments, the effect of duty cycle on theetch depth may still be significant even when the duty cycle is below50%, below 40%, or below 30% because the etch depth also depends on thegrating pitch or periods. For example, for gratings having a common dutycycle but different grating periods, the grating having a larger periodmay have a wider trench, which may lead to a deeper etched trench. Thus,as the grating period changes, the ME lag curve may start to drop at aduty cycle different from that shown in FIG. 14.

According to certain embodiments, to fabricate a nanostructure (e.g., amolded or etched slanted grating or a master mold for NIL) with a dutycycle range that includes large duty cycles (e.g., from about 0.5 toabout 0.9 as shown by a region 1420 in RIE lag curve 1410), an initialnanostructure with reduced duty cycles (e.g., from about 0.3 to about0.7 as shown by region 1430 in RIE lag curve 1410) may be imprinted oretched first, where the mask for the etching and/or the stamp for thenanoimprint may be adjusted to have duty cycles smaller (or larger for anegative working stamp) than the desired duty cycles of thenanostructure. One or more layers of materials may then be deposited onthe surfaces of the initial nanostructure to increase the duty cycles ofthe nanostructure. For example, one or more uniform layers of oxide(e.g., SiO₂, Al₂O₃, TiO₂, HfO₂, ZrO₂, ZnO₂, Si₃N₄, and the like) may beconformally deposited on the surfaces of the initial nanostructure usingtechniques such as atomic layer deposition (ALD) to increase the dutycycles of the nanostructure. In some embodiments, the materials of thedeposited layers may have refractive indices close to or higher than therefractive index of the imprinted or etched initial nanostructure.

FIG. 15 is a flow chart 1500 illustrating an example of a method forfabricating a surface-relief grating with large duty cycles according tocertain embodiments. The operations described in flow chart 1500 are forillustration purposes only and are not intended to be limiting. Invarious implementations, modifications may be made to flow chart 1500 toadd additional operations or to omit some operations.

At block 1510, an etch mask with duty cycles smaller than the desiredduty cycles may be patterned on a substrate. The substrate may include ametal, dielectric, semiconductor, or ceramic substrate, such as a metalalloy substrate, a silicon substrate, a SiO₂ layer, a Si₃N₄ materiallayer, a titanium oxide layer, an alumina layer, a SiC layer, aSiO_(x)N_(y) layer, an amorphous silicon layer, a spin on carbon (SOC)layer, an amorphous carbon layer (ACL), a diamond like carbon (DLC)layer, a TiO_(x) layer, an AlO_(x) layer, a TaO_(x) layer, or a HfO_(x)layer. The etch mask may include a photoresist mask or a hard mask, suchas a metal (e.g., copper, chrome, titanium, aluminum, or molybdenum) ora metallic compound (e.g., MoSiON) mask. The etch mask may be patternedon the substrate using various lithography techniques, such as aphotolithography process. The duty cycles of the pattern on the etchmask may be smaller than the desired duty cycles of the nanostructuresto be fabricated. For example, as described above, to make ananostructure with a duty cycle range that includes large duty cycles,such as from about 0.5 to about 0.9, the duty cycles of the pattern onthe etch mask may be reduced to, for example, from about 0.3 to about0.7, from about 0.25 to about 0.65, or the like, where the etch rate maybe relatively high and flat for the reduced duty cycle based on theactual RIE lag curve of the etching process.

At block 1520, the substrate may be etched using the etch mask to form ananostructure in the substrate. For example, the substrate may bevertically etched to form straight nanostructures, or may beslant-etched to form slanted nanostructure as described above, where theslant angle may be greater than about 10°, 20°, 30°, 45°, or 60°. Theetching may include an ion beam etching using various chemicals, such ashydrogen ions, helium ions, oxygen ions, and reactive gases (e.g., atleast one of CF₄, CHF₃, CH₂F₂, CH₃F, C₄F₈, C₄F₆, C₂F₆, C₂F₈, NF₃, CLF₃,N₂O, N₂, O₂, SO₂, COS, SF₆, Cl₂, BCl₃, HBr, H₂, Ar, He, or Ne). Becausethe etch mask has reduced duty cycles for which the etch rates may besubstantially high and flat, the nanostructure may be etched at a higherspeed and the etch depth after the etching process may be substantiallythe same for regions with different duty cycles. For example, adifference between the depth of a region having the minimum duty cycleand the depth of a region having the maximum duty cycle may be less than20%, 10%, 5%, or lower of the depth of the region having the minimumduty cycle. In some embodiments, the minimum depth of the nanostructureetched in the substrate may be greater than 100 nm, 200 nm, or 300 nm.

In some embodiments, at block 1530, a layer of material may beconformally deposited on surfaces of the nanostructure. The material mayinclude, for example, SiO₂, Al₂O₃, TiO₂, HfO₂, ZrO, ZnO₂, Si₃N₄, or thelike. In some embodiments, the layer of material may be conformallydeposited on the surfaces of the nanostructure by, for example, atomiclayer deposition (ALD) or plasma enhanced chemical vapor deposition(PECVD). In some embodiments, the thickness of the layer of material mayvary less than about 10%, 5%, 2%, 1%, or lower of the average thicknessof the layer of material. In some embodiments, the thickness of thelayer of material may be between about 2.5% to about 20% of the gratingperiod. As such, the duty cycles of the nanostructure may increase byabout 0.05 to about 0.4, due to the layer of material on each side of agrating ridge.

In the ALD process, the nanostructure may be exposed to, for example,two reactants, in a sequential, non-overlapping way. The ALD process canbe used to uniformly deposit low refractive index materials (e.g., Al₂O₃or SiO₂) or high refractive index materials (e.g., ZnS, HfO₂, or TiO₂)on exposed surfaces of the nanostructure. For example, the atomic layerdeposition of silicon dioxide (SiO₂) may be performed using a variety ofsilicon precursors and oxidants, such as H₂O, oxygen plasma, or O₃. Afirst reactant may react with the exposed surfaces in a self-limitedway, where the reactant molecules may react only with a finite number ofreactive sites on the surfaces. Once all the reactive sites have beenconsumed, the reaction (or deposition) may stop, and the remaining firstreactant molecules may be flushed away. A second reactant may then beinjected into the reactor after the first reactant has been flushedaway, and may only react with the first reactant at the finite number ofreactive sites on the surfaces. Because the reactants are injected intothe reactor at different times and excess reactants are purged before adifferent reactant is injected, the reaction does not take place in thegas phase, and is surface-limited. As such, ALD offers a highreproducibility, large-area thickness uniformity, and conformal coatingon structures with a high aspect ratio. The surface-controlled ALDgrowth mechanism enables the precise control of film thickness within asub-nanometer range or nanometer range as well as high repeatability.Using atomic layer deposition, the thickness of the resultant depositionlayer can be precisely controlled by the number of ALD cycles.

In some embodiments, different masks and different ALD processes (e.g.,cycles) may be used for the deposition of the layer of material, suchthat the thickness of the layer of material may be different atdifferent regions of the nanostructure. Thus, the duty cycles atdifferent regions of the nanostructure may be increased by differentvalues due to the selective atomic layer deposition. In someembodiments, different materials may be deposited in at least some ALDcycles such that the layer of material includes two or more layers ofdifferent materials. After the deposition of the layer of material, theduty cycles of the nanostructure may be increased to the desired values.The nanostructure made by operations at blocks 1510-1530 may be a finalproduct (a grating for near-eye display) or may be used as a master moldfor imprinting stamps or nanostructures.

In some embodiments where the nanostructure fabricated using operationsat blocks 1510-1530 is used as a master mold, surface-reliefnanostructures, such as surface-relief gratings or different generationsof stamps, may be imprinted in organic material layers at block 1540using the nanostructure in the substrate as described above with respectto FIGS. 8A-11.

In some embodiments where the nanostructure fabricated using operationsat blocks 1510 and 1520 is used as a master mold, surface-reliefnanostructures (e.g., surface-relief gratings or different generationsof stamps) imprinted in organic material layers at block 1540 using themaster mold may have duty cycles lower than the desired duty cycles. Atblock 1550, a layer of material may be conformally deposited on surfacesof the surface-relief nanostructure. The material may include, forexample, Sift, Al₂O₃, TiO₂, HfO₂, ZrO, ZnO₂, Si₃N₄, or the like. In someembodiments, the layer of material may be conformally deposited on thesurfaces of the surface-relief nanostructure by, for example, atomiclayer deposition (ALD) or plasma enhanced chemical vapor deposition(PECVD) as described above with respect to block 1530. For example, insome embodiments, multiple ALD cycles may be performed to deposit thelayer of material with a desired thickness. In some embodiments,different materials may be deposited in at least some ALD cycles suchthat the layer of material may include two or more layers of differentmaterials. In some embodiments, the thickness of the layer of materialmay vary less than about 10%, 5%, 2%, 1%, or lower of the averagethickness of the layer of material. In some embodiments, the thicknessof the layer of material may be between about 2.5% to about 20% of thegrating period. As such, the duty cycles of the surface-reliefnanostructure may increase by about 0.05 to about 0.4, due to the layerof material on each side of a grating ridge.

In some embodiments, at block 1560, one or more additional layers ofmaterials may be deposited on the surface-relief nanostructure. The oneor more additional layers of materials may include, for example, a layerof material with a higher refractive index, an overcoat layer, apatterned layer (e.g., an antireflective layer), or any combination. Insome embodiments, the surface-relief nanostructure may be over-coatedwith a material having a refractive index different from thesurface-relief nanostructure (e.g., the imprint resin layer or thesubstrate). For example, in some embodiments, a higher refractive indexmaterial, such as Hafnia, Titania, Tungsten oxide, Zirconium oxide,Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, or a highrefractive index polymer, may be used to over-coat the surface-reliefnanostructure and fill the gaps in the surface-relief nanostructure. Insome embodiments, a lower refractive index material, such as siliconoxide, magnesium fluoride, porous silica, or fluorinated low indexmonomer (or polymer), and the like, may be used to over-coat thesurface-relief nanostructure and fill the gaps in the surface-reliefnanostructure.

FIG. 16A illustrates an example of a slanted surface-relief grating 1620fabricated on a substrate 1610 using reactive ion etching ornanolithography according to certain embodiments. Surface-relief grating1620 may be fabricated after the operations at block 1520 or at block1540. Substrate 1610 may be any substrate as described above.Surface-relief grating 1620 may be a straight grating or a slantedgrating. As shown in the example, the configuration of surface-reliefgrating 1620 may vary across substrate 1610 so as to improve theperformance of the system using surface-relief grating 1620, such asincreasing the coupling efficiency of the light to user's eyes in awaveguide display system. For example, a region 1620 a of surface-reliefgrating 1620 may have a grating duty cycle DC1 that is different fromthe grating duty cycle DC2 of another region 1620 b of surface-reliefgrating 1620. Thus, the widths of grating ridges 1622 a and 1622 band/or the widths of the grating grooves 1624 a and 1624 b (and hencethe aspect ratio in regions 1620 a and 1620 b) may be different. Thegrating period in region 1620 a and the grating period in region 1620 bmay also be different.

As described above, the desired duty cycle at region 1620 a and thedesire duty cycle at region 1620 b of surface-relief grating 1620 may belarger than grating duty cycle DC1 and grating duty cycle DC2,respectively. For example, the desired duty cycle in region 1620 a maybe 0.6 and the desired duty cycle in region 1620 b may be 0.8, whilegrating duty cycle DC1 may be 0.5 and grating duty cycle DC2 may be 0.7(for which the RIE lag effect may be less prominent). Thus, the widthsof grating grooves 1624 a and 1624 b may be larger than the desiredwidth and the aspect ratios of grating grooves 1624 a and 1624 b may belower than the desired aspect ratio, while the widths of grating ridges1622 a and 1622 b may be lower than the desired width. As such, the MElag effect may be less prominent or the demolding process may be mucheasier in surface-relief grating 1620 due to the larger width of gratinggrooves 1624 a and 1624 b (i.e., lower duty cycles), and the desiredgrating depths may be relatively easy to achieve in both region 1620 aand region 1620 b using a same etching or imprint process, such asreactive ion etching or NIL.

FIG. 16B illustrates a first region 1620 a of surface-relief grating1620 of FIG. 16A that has been coated with a material layer 1630according to certain embodiments. Material layer 1630 may be conformallycoated on exposed surfaces of surface-relief grating 1620 as describedabove with respect to, for example, block 1530 or block 1550. Materiallayer 1630 may be substantially uniform in thickness on all exposedsurfaces of surface-relief grating 1620, and may be conformallydeposited on the surfaces of surface-relief grating 1620 by, forexample, one or more atomic layer deposition (ALD) cycles or plasmaenhanced chemical vapor deposition (PECVD) processes. For example, insome embodiments, the thickness of material layer 1630 may vary lessthan about 10%, 5%, 2%, 1%, or lower of the average thickness ofmaterial layer 1630. In some embodiments, the thickness of materiallayer 1630 may be between about 2.5% to about 20% of the grating period.In some embodiments, multiple ALD cycles may be performed in order toachieve the desired thickness. In some embodiments, different materialsmay be deposited in at least some ALD cycles such that material layer1630 may include two or more layers of different materials. As such,after the deposition of material layer 1630, the duty cycles of thenanostructure may increase by, for example, about 0.05 to about 0.4. Inthe example shown in FIG. 16B, the thickness of material layer 1630 maybe about 5% of the grating period. As such, the duty cycle DC1′ ofsurface-relief grating 1620 in first region 1620 a may increase, forexample, from about 0.4 to about 0.5 (i.e., by about 0.1), due to thelayer of material on each side of a grating ridge.

FIG. 16C illustrates a second region 1620 b of surface-relief grating1620 of FIG. 16A that has been coated with material layer 1630 accordingto certain embodiments. As described above, material layer 1630 may beconformally coated on exposed surfaces of surface-relief grating 1620 asdescribed above with respect to, for example, block 1530 or block 1550.Material layer 1630 may be substantially uniform in thickness on allexposed surfaces of surface-relief grating 1620, and may be conformallydeposited on the surfaces of surface-relief grating 1620 by, forexample, atomic layer deposition (ALD) or plasma enhanced chemical vapordeposition (PECVD). In some embodiments, multiple ALD cycles may beperformed in order to achieve a desired thickness. In some embodiments,different materials may be deposited in at least some ALD cycles suchthat material layer 1630 may include two or more layers of differentmaterials. For example, in some embodiments, the thickness of materiallayer 1630 may vary less than about 10%, 5%, 2%, 1%, or lower of theaverage thickness of material layer 1630. In some embodiments, thethickness of material layer 1630 may be between about 2.5% to about 20%of the grating period. As such, the duty cycles of the nanostructure mayincrease by, for example, about 0.05 to about 0.4. In the example shownin FIG. 16C, the thickness of material layer 1630 may be about 5% of thegrating period. As such, duty cycle DC2′ of surface-relief grating 1620in second region 1620 b may increase, for example, from about 0.7 toabout 0.8 (i.e., by about 0.1) due to the layer of material on each sideof a grating ridge.

As described above, in some embodiments, a nanostructure may beimprinted in an organic material layer using a master mold that haslarge duty cycles and is etched using techniques described above, or maybe directly etched in an organic material layer that may be etched muchfaster than inorganic materials, such as semiconductor, silicon dioxide,silicon nitride, titanium dioxide, alumina, ceramic, SiC, SiO_(x)N_(y),amorphous silicon, spin on carbon, amorphous carbon, diamond likecarbon, TiO_(x), AlO_(x), TaO_(x), HfO_(x), and the like. The organicmaterial may have a lower refractive index, such as about 1.5. Toincrease the effective refractive index of the nanostructure, one ormore sub-wavelength layers of a material having a high refractive indexmay be deposited on the surface of the nanostructure. For example, oneor more uniform layers of oxide or other high refractive index materials(e.g., Al₂O₃, TiO₂, HfO₂, ZrO₂, ZnO₂, Si₃N₄, and the like) may beconformally deposited on the surfaces of the initial nanostructure usingatomic layer deposition (ALD) techniques. In some embodiments, multipleALD cycles may be performed to deposit the one or more uniform layers ofhigh refractive index materials. In some embodiments, differentmaterials may be deposited in at least some ALD cycles such that the oneor more uniform layers of high refractive index materials may includetwo or more layers of different materials. The materials of thedeposited layers may have refractive indices higher than the refractiveindex of the imprinted or etched initial nanostructure (e.g., resin,polymer, or other organic materials). The thickness of the one or moredeposited layers may be significantly shorter than (e.g., a fraction of)the working wavelength of the nanostructure, such that the depositedlayer may not significantly change the physical dimensions and/orcertain optical performance of the nanostructure.

As also described above, in some embodiments, the surface-relief gratingmay be over-coated with a material having a refractive index differentfrom the surface-relief grating (e.g., the imprint resin layer or thesubstrate). For example, in some embodiments, a higher refractive indexmaterial, such as Hafnia, Titania, Tungsten oxide, Zirconium oxide,Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, or a highrefractive index polymer, may be used to over-coat the slanted gratingand fill the gaps between the slanted grating ridges. In someembodiments, a lower refractive index material, such as silicon oxide,magnesium fluoride, porous silica, or fluorinated low index monomer (orpolymer), and the like, may be used to over-coat the slanted grating andfill the gaps between the slanted grating ridges.

The materials that can be deposited on the imprinted nanostructures(e.g., surface-relief gratings) may generally have refractive indicesdifferent from the refractive index of the imprinted nanostructure(e.g., comprising of polymers). In some cases, the difference inrefractive index may change the optical performance of thenanostructures, such as increasing the losses at the interface betweendifferent materials (e.g., due to Fresnel reflection or total internalreflection) or changing the diffraction properties of the surface-reliefgratings, in particular, when the deposited layer is relatively thickand when there may be many layers of different materials. Thus, thenanostructure with the deposited ALD layers may have increased dutycycles, but may have different optical performance than a nanostructurehaving the same duty cycles but made using a same material.

FIG. 17A illustrates an example of a slanted surface-relief structure1700 fabricated using nanoimprint lithography and coated with a firstmaterial layer 1730 according to certain embodiments. Surface-reliefstructure 1700 may include a surface-relief grating 1720 imprinted in animprint material layer 1710 using the NIL techniques according tocertain embodiments described above. Surface-relief grating 1720 may befabricated by the operations at block 1540. Surface-relief grating 1720may be a straight grating or a slanted grating. In the example shown inFIG. 17A, surface-relief grating 1720 may include a plurality of slantedgrating ridges 1722 and a plurality of grating grooves 1724. The widthand/or slant angle of each grating ridge 1722, and the width and/ordepth of each grating groove 1724 may be different at different regionsof surface-relief grating 1720. Imprinted material layer 1710 mayinclude a resin material that has a refractive index between, forexample, about 1.5 and about 1.8, which may be a function of the amountof high refractive index nanoparticles in the resin material.

First material layer 1730 may be deposited on surface-relief grating1720 using, for example, ALD processes. First material layer 1730 mayhave a subwavelength thickness (e.g., tens of nanometers or hundreds ofnanometers) and may have an refractive index lower or higher than therefractive index of the resin material in grating ridges 1722. In orderto match the refractive index of the resin material, a thin secondmaterial layer with a refractive index higher or lower than therefractive index of the resin material may be deposited on firstmaterial layer 1730 such that the combined material layers may have aneffective refractive index matching the refractive index of the resinmaterial.

FIG. 17B illustrates the slanted surface-relief structure 1700 of FIG.17A that has been coated with a second material layer 1740 according tocertain embodiments. Second material layer 1740 may be deposited onfirst material layer 1730 and may have a refractive index different fromthat of first material layer 1730. The thicknesses of second materiallayer 1740 and the thickness of first material layer 1730 may beselected based on the refractive indices of first material layer 1730,second material layer 1740, and the resin material in grating ridges1722.

In one example, first material layer 1730 may have a refractive indexlower than the refractive index of the resin material and secondmaterial layer 1740 may have a refractive index higher than therefractive index of the resin material. If the difference between therefractive index of first material layer 1730 and the refractive indexof the resin material is greater than the difference between therefractive index of second material layer 1740 and the refractive indexof the resin material, second material layer 1740 may be thicker thanfirst material layer 1730, such that the overall effective refractiveindex of the combination of first material layer 1730 and secondmaterial layer 1740 may be close to the refractive index of the resinmaterial of grating ridges 1722. Because first material layer 1730 andsecond material layer 1740 may be deposited using ALD processes, thethicknesses of first material layer 1730 and second material layer 1740can be precisely controlled, and thus the effective refractive index ofthe combination of first material layer 1730 and second material layer1740 can precisely match the refractive index of the resin material.

In some embodiments, to reduce the effect of the different refractiveindices between grating ridges 1722 and first material layer 1730 andthe different refractive indices between first material layer 1730 andsecond material layer 1740 on the performance of the surface-reliefgrating, first material layer 1730 and second material layer 1740 mayneed to be thin, such as about tens of nanometers or thinner, which maynot be sufficient to increase the duty cycle by the desired value.According to certain embodiments, a plurality of thin material layersmay be used to achieve both the refractive index matching and thedesired duty cycle increase, while reducing the effect of the differentmaterials on the performance of the surface-relief grating.

FIG. 17C illustrates an example of a stack of coating layers matchingthe refractive index of an imprinted surface-relief grating (e.g.,surface-relief grating 1720) according to certain embodiments. In theexample shown in FIG. 17C, the coating layers may include a stack ofthin layers of two alternate materials, such as the two materials infirst material layer 1730 and second material layer 1740 describedabove. For example, as shown in FIG. 17C, the stack of thin layers mayinclude n groups of layers, where a first group of layers may include athin layer 1730-1 of a first material having a lower refractive indexand a thin layer 1740-1 of a second material having a higher refractiveindex, . . . , and the nth group of layers may include a thin layer1730-n of the first material and a thin layer 1740-n of the secondmaterial. Each of the layers in the stack may have a thickness of, forexample, a few nanometers or a few tens of nanometers.

In some embodiments, the materials for the stack of layers may includethree or more different materials and the layers of the differentmaterials can be arranged in any suitable manners to achieve therefractive index matching and the desired duty cycle increase. In someembodiments, the layers may have the same or different thicknesses.

In some applications, alternatively or in addition to increasing theduty cycles, it may be desirable to modify the refractive indexmodulation of the imprinted surface-relief grating, such as increasingor apodizing the refractive index modulation of the imprintedsurface-relief grating. For example, increasing the refractive indexmodulation of the imprinted surface-relief grating may increase thediffraction efficiency and increase the angular bandwidth of theimprinted surface-relief grating.

FIG. 18A illustrates an example of a slanted surface-relief structure1800 fabricated using nanoimprint lithography and coated with one ormore thin material layers 1830 according to certain embodiments.Surface-relief structure 1800 may include a surface-relief grating 1820imprinted in an imprint material layer 1810 using the NIL techniquesaccording to certain embodiments described above. For example,surface-relief grating 1820 may be fabricated by the operations at block1540. Surface-relief grating 1820 may be a straight grating or a slantedgrating. In the example shown in FIG. 18A, surface-relief grating 1820may include a plurality of slanted grating ridges 1822 and a pluralityof grating grooves 1824. The width and/or slant angle of each gratingridge 1822, and the width and/or depth of each grating groove 1824 maybe different at different regions of surface-relief grating 1820.Imprinted material layer 1810 may include a resin material that has arefractive index between, for example, from about 1.5 to about 1.8,which may be a function of the amount of high refractive indexnanoparticles in the resin material as described above. In the exampleshown in FIG. 18A, the one or more thin material layers 1830 depositedon surface-relief grating 1820 may have a refractive index higher thanthe refractive index of grating ridges 1822.

FIG. 18B illustrates slanted surface-relief structure 1800 of FIG. 18Athat has been coated with an overcoat layer 1840 according to certainembodiments. Overcoat layer 1840 may fill grating grooves 1824. Overcoatlayer 1840 may include a material having a refractive index higher thanthe refractive index of one or more thin material layers 1830 or mayinclude a material having a refractive index lower than the refractiveindex of grating ridges 1822. In one example, the refractive index ofgrating ridges 1822 may be about 1.6, the refractive index of materiallayers 1830 may be about 1.8, and the refractive index of overcoat layer1840 may be about 2.2. As such, region 1820 a of surface-reliefstructure 1800 may include a grating with a refractive index modulationof about 0.2 (e.g., 1.8−1.6). Region 1820 b of surface-relief structure1800 may include a grating with a refractive index modulation of about0.4 (e.g., (1.8+2.2)/2−1.6). Region 1820 c of surface-relief structure1800 may include a grating with a refractive index modulation of about0.4 (e.g., 2.2−1.8). In some embodiments, the diffractions by themultiple gratings in regions 1820 a, 1820 b, 1820 c, and the like mayinterfere with each other to reduce certain artifacts, such as therainbow effect.

FIG. 19A illustrates another example of a slanted surface-reliefstructure 1900 fabricated using nanoimprint lithography and coated withone or more thin material layers 1930 according to certain embodiments.Surface-relief structure 1900 may include a surface-relief grating 1920imprinted in an imprint material layer 1910 using the NIL techniquesaccording to certain embodiments described above. Surface-relief grating1920 may be a straight grating or a slanted grating. In the exampleshown in FIG. 19A, surface-relief grating 1920 may include a pluralityof slanted grating ridges 1922 and a plurality of grating grooves 1924.The width and/or slant angle of each grating ridge 1922, and the widthand/or depth of each grating groove 1924 may be different at differentregions of surface-relief grating 1920. Imprinted material layer 1910may include a resin material that has a first refractive index, such asbetween about 1.5 and about 1.8. In the example shown in FIG. 19A, theone or more thin material layers 1930 deposited on surface-reliefgrating 1920 may have a second refractive index lower than the firstrefractive index.

FIG. 19B illustrates the slanted surface-relief structure 1900 of FIG.19A that has been coated with an overcoat layer 1940 according tocertain embodiments. Overcoat layer 1940 may fill grating grooves 1924.Overcoat layer 1940 may include a material having a refractive indexhigher or lower than the refractive index of one or more thin materiallayers 1930 and/or the refractive index of grating ridges 1922. In oneexample, the refractive index of grating ridges 1922 may be about 1.8,the refractive index of material layers 1930 may be about 1.7, and therefractive index of overcoat layer 1940 may be about 1.5. As such,region 1920 a of surface-relief structure 1900 may include a gratingwith a refractive index modulation of about 0.1 (e.g., 1.8−1.7). Region1920 b of surface-relief structure 1900 may include a grating with arefractive index modulation of about 0.2 (e.g., (1.8−(1.7+1.5)/2).Region 1920 c of surface-relief structure 1900 may include a gratingwith a refractive index modulation of about 0.2 (e.g., 1.7−1.5). In someembodiments, the diffractions by the multiple gratings in regions 1920a, 1920 b, 1920 c, and the like may interfere with each other to reducecertain artifacts, such as the rainbow effect.

As described above, in some embodiments, multiple material layers may bedeposited on the imprinted surface-relief grating (e.g., surface-reliefgrating 1920), where the multiple material layers may have differentrefractive indices. In some embodiments, the refractive indices of themultiple material layers may gradually decrease or increase from thegrating ridges to the overcoat layer, such that the refractive indexmodulation may gradually decrease to zero to form an apodized grating.

Embodiments of the invention may be used to implement components of anartificial reality system or may be implemented in conjunction with anartificial reality system. Artificial reality is a form of reality thathas been adjusted in some manner before presentation to a user, whichmay include, for example, a virtual reality (VR), an augmented reality(AR), a mixed reality (MR), a hybrid reality, or some combination and/orderivatives thereof. Artificial reality content may include completelygenerated content or generated content combined with captured (e.g.,real-world) content. The artificial reality content may include video,audio, haptic feedback, or some combination thereof, and any of whichmay be presented in a single channel or in multiple channels (such asstereo video that produces a three-dimensional effect to the viewer).Additionally, in some embodiments, artificial reality may also beassociated with applications, products, accessories, services, or somecombination thereof, that are used to, for example, create content in anartificial reality and/or are otherwise used in (e.g., performactivities in) an artificial reality. The artificial reality system thatprovides the artificial reality content may be implemented on variousplatforms, including a head-mounted display (HMD) connected to a hostcomputer system, a standalone HMD, a mobile device or computing system,or any other hardware platform capable of providing artificial realitycontent to one or more viewers.

FIG. 20 is a simplified block diagram of an example electronic system2000 of an example near-eye display (e.g., HMD device) for implementingsome of the examples disclosed herein. Electronic system 2000 may beused as the electronic system of an HMD device or other near-eyedisplays described above. In this example, electronic system 2000 mayinclude one or more processor(s) 2010 and a memory 2020. Processor(s)2010 may be configured to execute instructions for performing operationsat a number of components, and can be, for example, a general-purposeprocessor or microprocessor suitable for implementation within aportable electronic device. Processor(s) 2010 may be communicativelycoupled with a plurality of components within electronic system 2000. Torealize this communicative coupling, processor(s) 2010 may communicatewith the other illustrated components across a bus 2040. Bus 2040 may beany subsystem adapted to transfer data within electronic system 2000.Bus 2040 may include a plurality of computer buses and additionalcircuitry to transfer data.

Memory 2020 may be coupled to processor(s) 2010. In some embodiments,memory 2020 may offer both short-term and long-term storage and may bedivided into several units. Memory 2020 may be volatile, such as staticrandom access memory (SRAM) and/or dynamic random access memory (DRAM)and/or non-volatile, such as read-only memory (ROM), flash memory, andthe like. Furthermore, memory 2020 may include removable storagedevices, such as secure digital (SD) cards. Memory 2020 may providestorage of computer-readable instructions, data structures, programmodules, and other data for electronic system 2000. In some embodiments,memory 2020 may be distributed into different hardware modules. A set ofinstructions and/or code might be stored on memory 2020. Theinstructions might take the form of executable code that may beexecutable by electronic system 2000, and/or might take the form ofsource and/or installable code, which, upon compilation and/orinstallation on electronic system 2000 (e.g., using any of a variety ofgenerally available compilers, installation programs,compression/decompression utilities, etc.), may take the form ofexecutable code.

In some embodiments, memory 2020 may store a plurality of applicationmodules 2022 through 2024, which may include any number of applications.Examples of applications may include gaming applications, conferencingapplications, video playback applications, or other suitableapplications. The applications may include a depth sensing function oreye tracking function. Application modules 2022-2024 may includeparticular instructions to be executed by processor(s) 2010. In someembodiments, certain applications or parts of application modules2022-1524 may be executable by other hardware modules 2080. In certainembodiments, memory 2020 may additionally include secure memory, whichmay include additional security controls to prevent copying or otherunauthorized access to secure information.

In some embodiments, memory 2020 may include an operating system 2025loaded therein. Operating system 2025 may be operable to initiate theexecution of the instructions provided by application modules 2022-2024and/or manage other hardware modules 2080 as well as interfaces with awireless communication subsystem 2030 which may include one or morewireless transceivers. Operating system 2025 may be adapted to performother operations across the components of electronic system 2000including threading, resource management, data storage control, andother similar functionality.

Wireless communication subsystem 2030 may include, for example, aninfrared communication device, a wireless communication device and/orchipset (such as a Bluetooth® device, an IEEE 802.11 device, a Wi-Fidevice, a WiMax device, cellular communication facilities, etc.), and/orsimilar communication interfaces. Electronic system 2000 may include oneor more antennas 2034 for wireless communication as part of wirelesscommunication subsystem 2030 or as a separate component coupled to anyportion of the system. Depending on desired functionality, wirelesscommunication subsystem 2030 may include separate transceivers tocommunicate with base transceiver stations and other wireless devicesand access points, which may include communicating with different datanetworks and/or network types, such as wireless wide-area networks(WWANs), wireless local area networks (WLANs), or wireless personal areanetworks (WPANs). A WWAN may be, for example, a WiMax (IEEE 802.16)network. A WLAN may be, for example, an IEEE 802.11x network. A WPAN maybe, for example, a Bluetooth network, an IEEE 802.15x, or some othertypes of network. The techniques described herein may also be used forany combination of WWAN, WLAN, and/or WPAN. Wireless communicationssubsystem 2030 may permit data to be exchanged with a network, othercomputer systems, and/or any other devices described herein. Wirelesscommunication subsystem 2030 may include a means for transmitting orreceiving data, such as identifiers of HMD devices, position data, ageographic map, a heat map, photos, or videos, using antenna(s) 2034 andwireless link(s) 2032. Wireless communication subsystem 2030,processor(s) 2010, and memory 2020 may together comprise at least a partof one or more of a means for performing some functions disclosedherein.

Embodiments of electronic system 2000 may also include one or moresensors 2090. Sensor(s) 2090 may include, for example, an image sensor,an accelerometer, a pressure sensor, a temperature sensor, a proximitysensor, a magnetometer, a gyroscope, an inertial sensor (e.g., a modulethat combines an accelerometer and a gyroscope), an ambient lightsensor, or any other similar module operable to provide sensory outputand/or receive sensory input, such as a depth sensor or a positionsensor. For example, in some implementations, sensor(s) 2090 may includeone or more inertial measurement units (IMUs) and/or one or moreposition sensors. An IMU may generate calibration data indicating anestimated position of the HMD device relative to an initial position ofthe HMD device, based on measurement signals received from one or moreof the position sensors. A position sensor may generate one or moremeasurement signals in response to motion of the HMD device. Examples ofthe position sensors may include, but are not limited to, one or moreaccelerometers, one or more gyroscopes, one or more magnetometers,another suitable type of sensor that detects motion, a type of sensorused for error correction of the IMU, or some combination thereof. Theposition sensors may be located external to the IMU, internal to theIMU, or some combination thereof. At least some sensors may use astructured light pattern for sensing.

Electronic system 2000 may include a display module 2060. Display module2060 may be a near-eye display, and may graphically present information,such as images, videos, and various instructions, from electronic system2000 to a user. Such information may be derived from one or moreapplication modules 2022-2024, virtual reality engine 2026, one or moreother hardware modules 2080, a combination thereof, or any othersuitable means for resolving graphical content for the user (e.g., byoperating system 2025). Display module 2060 may use liquid crystaldisplay (LCD) technology, light-emitting diode (LED) technology(including, for example, OLED, ILED, μLED, AMOLED, TOLED, etc.), lightemitting polymer display (LPD) technology, or some other displaytechnology.

Electronic system 2000 may include a user input/output module 2070. Userinput/output module 2070 may allow a user to send action requests toelectronic system 2000. An action request may be a request to perform aparticular action. For example, an action request may be to start or endan application or to perform a particular action within the application.User input/output module 2070 may include one or more input devices.Example input devices may include a touchscreen, a touch pad,microphone(s), button(s), dial(s), switch(es), a keyboard, a mouse, agame controller, or any other suitable device for receiving actionrequests and communicating the received action requests to electronicsystem 2000. In some embodiments, user input/output module 2070 mayprovide haptic feedback to the user in accordance with instructionsreceived from electronic system 2000. For example, the haptic feedbackmay be provided when an action request is received or has beenperformed.

Electronic system 2000 may include a camera 2050 that may be used totake photos or videos of a user, for example, for tracking the user'seye position. Camera 2050 may also be used to take photos or videos ofthe environment, for example, for VR, AR, or MR applications. Camera2050 may include, for example, a complementary metal-oxide-semiconductor(CMOS) image sensor with a few millions or tens of millions of pixels.In some implementations, camera 2050 may include two or more camerasthat may be used to capture 3D images.

In some embodiments, electronic system 2000 may include a plurality ofother hardware modules 2080. Each of other hardware modules 2080 may bea physical module within electronic system 2000. While each of otherhardware modules 2080 may be permanently configured as a structure, someof other hardware modules 2080 may be temporarily configured to performspecific functions or temporarily activated. Examples of other hardwaremodules 2080 may include an audio output and/or input module (e.g., amicrophone or speaker), a near field communication (NFC) module, arechargeable battery, a battery management system, a wired/wirelessbattery charging system, etc. In some embodiments, one or more functionsof other hardware modules 2080 may be implemented in software.

In some embodiments, memory 2020 of electronic system 2000 may alsostore a virtual reality engine 2026. Virtual reality engine 2026 mayexecute applications within electronic system 2000 and receive positioninformation, acceleration information, velocity information, predictedfuture positions, or some combination thereof of the HMD device from thevarious sensors. In some embodiments, the information received byvirtual reality engine 2026 may be used for producing a signal (e.g.,display instructions) to display module 2060. For example, if thereceived information indicates that the user has looked to the left,virtual reality engine 2026 may generate content for the HMD device thatmirrors the user's movement in a virtual environment. Additionally,virtual reality engine 2026 may perform an action within an applicationin response to an action request received from user input/output module2070 and provide feedback to the user. The provided feedback may bevisual, audible, or haptic feedback. In some implementations,processor(s) 2010 may include one or more graphic processing units(GPUs) that may execute virtual reality engine 2026.

In various implementations, the above-described hardware and modules maybe implemented on a single device or on multiple devices that cancommunicate with one another using wired or wireless connections. Forexample, in some implementations, some components or modules, such asGPUs, virtual reality engine 2026, and applications (e.g., trackingapplication), may be implemented on a console separate from thehead-mounted display device. In some implementations, one console may beconnected to or support more than one HMD.

In alternative configurations, different and/or additional componentsmay be included in electronic system 2000. Similarly, functionality ofone or more of the components can be distributed among the components ina manner different from the manner described above. For example, in someembodiments, electronic system 2000 may be modified to include othersystem environments, such as an AR system environment and/or an MRenvironment.

The methods, systems, and devices discussed above are examples. Variousembodiments may omit, substitute, or add various procedures orcomponents as appropriate. For instance, in alternative configurations,the methods described may be performed in an order different from thatdescribed, and/or various stages may be added, omitted, and/or combined.Also, features described with respect to certain embodiments may becombined in various other embodiments. Different aspects and elements ofthe embodiments may be combined in a similar manner. Also, technologyevolves and, thus, many of the elements are examples that do not limitthe scope of the disclosure to those specific examples.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, embodiments may be practicedwithout these specific details. For example, well-known circuits,processes, systems, structures, and techniques have been shown withoutunnecessary detail in order to avoid obscuring the embodiments. Thisdescription provides example embodiments only, and is not intended tolimit the scope, applicability, or configuration of the invention.Rather, the preceding description of the embodiments will provide thoseskilled in the art with an enabling description for implementing variousembodiments. Various changes may be made in the function and arrangementof elements without departing from the spirit and scope of the presentdisclosure.

Also, some embodiments were described as processes depicted as flowdiagrams or block diagrams. Although each may describe the operations asa sequential process, many of the operations may be performed inparallel or concurrently. In addition, the order of the operations maybe rearranged. A process may have additional steps not included in thefigure. Furthermore, embodiments of the methods may be implemented byhardware, software, firmware, middleware, microcode, hardwaredescription languages, or any combination thereof. When implemented insoftware, firmware, middleware, or microcode, the program code or codesegments to perform the associated tasks may be stored in acomputer-readable medium such as a storage medium. Processors mayperform the associated tasks.

It will be apparent to those skilled in the art that substantialvariations may be made in accordance with specific requirements. Forexample, customized or special-purpose hardware might also be used,and/or particular elements might be implemented in hardware, software(including portable software, such as applets, etc.), or both. Further,connection to other computing devices such as network input/outputdevices may be employed.

With reference to the appended figures, components that can includememory can include non-transitory machine-readable media. The term“machine-readable medium” and “computer-readable medium” may refer toany storage medium that participates in providing data that causes amachine to operate in a specific fashion. In embodiments providedhereinabove, various machine-readable media might be involved inproviding instructions/code to processing units and/or other device(s)for execution. Additionally or alternatively, the machine-readable mediamight be used to store and/or carry such instructions/code. In manyimplementations, a computer-readable medium is a physical and/ortangible storage medium. Such a medium may take many forms, including,but not limited to, non-volatile media, volatile media, and transmissionmedia. Common forms of computer-readable media include, for example,magnetic and/or optical media such as compact disk (CD) or digitalversatile disk (DVD), punch cards, paper tape, any other physical mediumwith patterns of holes, a RAM, a programmable read-only memory (PROM),an erasable programmable read-only memory (EPROM), a FLASH-EPROM, anyother memory chip or cartridge, a carrier wave as described hereinafter,or any other medium from which a computer can read instructions and/orcode. A computer program product may include code and/ormachine-executable instructions that may represent a procedure, afunction, a subprogram, a program, a routine, an application (App), asubroutine, a module, a software package, a class, or any combination ofinstructions, data structures, or program statements.

Those of skill in the art will appreciate that information and signalsused to communicate the messages described herein may be representedusing any of a variety of different technologies and techniques. Forexample, data, instructions, commands, information, signals, bits,symbols, and chips that may be referenced throughout the abovedescription may be represented by voltages, currents, electromagneticwaves, magnetic fields or particles, optical fields or particles, or anycombination thereof.

Terms, “and” and “or” as used herein, may include a variety of meaningsthat are also expected to depend at least in part upon the context inwhich such terms are used. Typically, “or” if used to associate a list,such as A, B, or C, is intended to mean A, B, and C, here used in theinclusive sense, as well as A, B, or C, here used in the exclusivesense. In addition, the term “one or more” as used herein may be used todescribe any feature, structure, or characteristic in the singular ormay be used to describe some combination of features, structures, orcharacteristics. However, it should be noted that this is merely anillustrative example and claimed subject matter is not limited to thisexample. Furthermore, the term “at least one of” if used to associate alist, such as A, B, or C, can be interpreted to mean any combination ofA, B, and/or C, such as A, AB, AC, BC, AA, ABC, AAB, AABBCCC, etc.

Further, while certain embodiments have been described using aparticular combination of hardware and software, it should be recognizedthat other combinations of hardware and software are also possible.Certain embodiments may be implemented only in hardware, or only insoftware, or using combinations thereof. In one example, software may beimplemented with a computer program product containing computer programcode or instructions executable by one or more processors for performingany or all of the steps, operations, or processes described in thisdisclosure, where the computer program may be stored on a non-transitorycomputer readable medium. The various processes described herein can beimplemented on the same processor or different processors in anycombination.

Where devices, systems, components or modules are described as beingconfigured to perform certain operations or functions, suchconfiguration can be accomplished, for example, by designing electroniccircuits to perform the operation, by programming programmableelectronic circuits (such as microprocessors) to perform the operationsuch as by executing computer instructions or code, or processors orcores programmed to execute code or instructions stored on anon-transitory memory medium, or any combination thereof. Processes cancommunicate using a variety of techniques, including, but not limitedto, conventional techniques for inter-process communications, anddifferent pairs of processes may use different techniques, or the samepair of processes may use different techniques at different times.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that additions, subtractions, deletions, and other modificationsand changes may be made thereunto without departing from the broaderspirit and scope as set forth in the claims. Thus, although specificembodiments have been described, these are not intended to be limiting.Various modifications and equivalents are within the scope of thefollowing claims.

What is claimed is:
 1. A surface-relief structure comprising: asurface-relief grating including a first material characterized by afirst refractive index; a first layer of a second material conformallydeposited on surfaces of the surface-relief grating, the second materialcharacterized by a second refractive index; and a second layer of athird material conformally deposited on the first layer, the thirdmaterial characterized by a third refractive index, wherein: one of thesecond refractive index and the third refractive index is lower orgreater than the first refractive index; and an effective refractiveindex of a combination of the first layer and the second layer is equalto the first refractive index.
 2. The surface-relief structure of claim1, wherein the surface-relief grating includes a grating imprinted in anorganic material.
 3. The surface-relief structure of claim 1, wherein amaximum duty cycle of the surface-relief structure is greater than 0.7.4. The surface-relief structure of claim 1, wherein a slant angle of thesurface-relief grating is greater than 30°.
 5. The surface-reliefstructure of claim 1, wherein a depth of the surface-relief grating isgreater than 100 nm.
 6. The surface-relief structure of claim 1, whereinthe first layer is characterized by a thickness less than 50 nm.
 7. Thesurface-relief structure of claim 1, wherein a thickness of the firstlayer and a thickness of the second layer are selected based on thefirst refractive index, the second refractive index, and the thirdrefractive index.
 8. The surface-relief structure of claim 1, furthercomprising: a third layer of the second material conformally depositedon the second layer; and a fourth layer of the third materialconformally deposited on the third layer.
 9. The surface-reliefstructure of claim 1, further comprising an overcoat layer on the secondlayer, the overcoat layer filling gaps in the surface-relief grating andcharacterized by a fourth refractive index different from the firstrefractive index.
 10. The surface-relief structure of claim 1, whereinthe first layer and the second layer are deposited by atomic layerdeposition or plasma enhanced chemical vapor deposition (PECVD).
 11. Thesurface-relief structure of claim 1, wherein the first layer ischaracterized by a variation in thickness less than 10% of an averagethickness of the first layer.
 12. The surface-relief structure of claim1, wherein the second material includes an oxide.
 13. A surface-reliefstructure comprising: a surface-relief grating including a firstmaterial characterized by a first refractive index; a first layer of asecond material conformally deposited on surfaces of the surface-reliefgrating, the second material characterized by a second refractive indexgreater than the first refractive index; and a second layer of a thirdmaterial conformally deposited on the first layer, the third materialcharacterized by a third refractive index greater than the secondrefractive index.
 14. The surface-relief structure of claim 13, whereinthe surface-relief grating includes a grating imprinted in an organicmaterial.
 15. The surface-relief structure of claim 13, wherein amaximum duty cycle of the surface-relief structure is greater than 0.7.16. The surface-relief structure of claim 13, further comprising anovercoat layer on the second layer, the overcoat layer filling gaps inthe surface-relief grating and characterized by a fourth refractiveindex greater than or equal to the third refractive index.
 17. Thesurface-relief structure of claim 13, wherein a thickness of the firstlayer is less than 50 nm.
 18. The surface-relief structure of claim 13,wherein the first layer and the second layer are deposited by atomiclayer deposition or plasma enhanced chemical vapor deposition (PECVD).19. A surface-relief structure comprising: a surface-relief gratingincluding a first organic material characterized by a first refractiveindex; a first layer of a second material conformally deposited onsurfaces of the surface-relief grating, the second materialcharacterized by a second refractive index lower than the firstrefractive index; and a second layer of a third material conformallydeposited on the first layer, the third material characterized by athird refractive index lower than the second refractive index.
 20. Thesurface-relief structure of claim 19, further comprising an overcoatlayer on the second layer, the overcoat layer filling gaps in thesurface-relief grating and characterized by a fourth refractive indexlower than or equal to the third refractive index.